qemu_process.c: removing qemuProcessStartValidateXML

Commit ("qemu_domain.c: moving maxCpu validation to
qemuDomainDefValidate") shortened the code of qemuProcessStartValidateXML.
The function is called only by qemuProcessStartValidate, in the
same file, and its code is now a single check that calls virDomainDefValidate.

Instead of leaving a function call just to execute a single check,
this patch puts the check in the body of qemuProcessStartValidate in the
place where qemuProcessStartValidateXML was being called. The function can
now be removed.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>