Here the new virtual cpu functions proposal.
What do you think about it ?



/**
 * virVcpuInfo: structure for information about a virtual CPU in a domain.
 */
typedef enum {
    VIR_VCPU_OFFLINE    = 0,    /* the virtual CPU is offline */
    VIR_VCPU_RUNNING    = 1,    /* the virtual CPU is running */
    VIR_VCPU_BLOCKED    = 2,    /* the virtual CPU is blocked on resource */
} virVcpuState;

typedef struct _virVcpuInfo virVcpuInfo;
struct _virVcpuInfo {
    unsigned int number;        /* virtual CPU number */
    int state;                  /* value from virVcpuState */
    unsigned long long cpuTime; /* CPU time used, in nanoseconds */
    int cpu;                    /* real CPU number, or -1 if offline */
};
typedef virVcpuInfo *virVcpuInfoPtr;


/**
 * virDomainSetVcpus:
 * @domain: pointer to domain object, or NULL for Domain0
 * @nvcpus: the new number of virtual CPUs for this domain
 *
 * Dynamically change the number of virtual CPUs used by the domain.
 * Note that this call may fail if the underlying virtualization hypervisor
 * does not support it or if growing the number is arbitrary limited.
 * This function requires priviledged access to the hypervisor.
 *
 * Returns 0 in case of success, -1 in case of failure.
 */
int virDomainSetVcpus   (virDomainPtr domain,
                         unsigned int nvcpus);


/**
 * virDomainPinVcpu:
 * @domain: pointer to domain object, or NULL for Domain0
 * @vcpu: virtual CPU number
 * @cpumap: pointer to a bit map of real CPUs (in 8-bit bytes).
 *      Each bit set to 1 means that corresponding CPU is usable.
 *      Bytes are stored in little-endian order: CPU0-7, 8-15...
 *      In each byte, lowest CPU number is least significant bit.
 * @maplen: number of bytes in cpumap, from 1 up to size of CPU map in
 *      underlying virtualization system (Xen...).
 *      If maplen < size, missing bytes are set to zero.
 *      If maplen > size, failure code is returned.
 *
 * Dynamically change the real CPUs which can be allocated to a virtual CPU.
 * This function requires priviledged access to the hypervisor.
 *
 * Returns 0 in case of success, -1 in case of failure.
 */
int virDomainPinVcpu    (virDomainPtr domain,
                         unsigned int vcpu,
                         unsigned char *cpumap,
                         int maplen);

/* Macros for bit manipulation in cpumap */
#define USE_CPU(cpumap,cpu)     (cpumap[(cpu)/8] |=  (1<<((cpu)%8)))
#define UNUSE_CPU(cpumap,cpu)   (cpumap[(cpu)/8] &= ~(1<<((cpu)%8)))


/**
 * virDomainGetVcpus:
 * @domain: pointer to domain object, or NULL for Domain0
 * @info: pointer to an array of virVcpuInfo structures
 * @maxinfo: number of structures in info array
 * @cpumaps: pointer to a bit map of real CPUs for all vcpus of this domain.
 *      If cpumaps is NULL, then no cpumap information is returned by the API.
 *      It's assumed there is <maxinfo> cpumap in cpumaps.
 *      The memory allocated to cpumaps must be (maxinfo * maplen) bytes.
 *      One cpumap inside cupmaps have the format described in virDomainPinVcpu API.
 * @maplen: number of bytes in one cpumap, from 1 up to size of CPU map in
 *      underlying virtualization system (Xen...).
 *
 * Extract information about virtual CPUs of domain, store it in info array.
 *
 * Returns the number of info filled in case of success, -1 in case of failure.
 */
int virDomainGetVcpus   (virDomainPtr domain,
                         virVcpuInfoPtr info,
                         int maxinfo,
                         unsigned char *cpumaps, /* may be NULL */
                         int maplen);

/* Macros for bit testing in cpumaps */
#define CPU_USABLE(cpumaps,maplen,vcpu,cpu) \
    (cpumaps[((vcpu)*(maplen))+((cpu)/8)] & (1<<((cpu)%8)))

/*
 * Macro for copying the cpumap of a vcpu from cupmaps inside a standalone cpumap.
 * This macro is useful in case of using virDomainPinVcpu() after virDomainGetVcpus().
 * cpumap must be previously allocated.
 */
#define COPY_CPUMAP(cpumaps,maplen,vcpu,cpumap) \
    memcpy(cpumap, &(cpumaps[(vcpu)*(maplen)]), (maplen))