qemu: Fix query-cpus-fast target architecture detection

Since qemu 2.13 reports the target architecture in a property called
'target' additionally to the property 'arch', that has been used in
qemu 2.12 in the response data of 'query-cpus-fast'.
Libvirts monitor code prefers the 'target' property over 'arch'.

At least for s390(x), target is reported as 's390x' while arch is 's390'.
In a later step a comparison is performed against 's390' which fails for
qemu 2.13 and later.

In consequence the architecture specific data for s390 won't be extracted
from the returned data, leading to incorrect values being reported by
virsh domstats --vcpu.

Changing to check explicitly for 's390' and 's390x'.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>