virsh: use correct sizeof when allocating cpumap

virsh: use correct sizeof when allocating cpumap

Found by coverity:
Error: SIZEOF_MISMATCH (CWE-569):
    libvirt-0.10.2/tools/virsh-domain.c:4754: suspicious_sizeof: Passing
    argument "8UL /* sizeof (cpumap) */" to function
    "_vshCalloc(vshControl *, size_t, size_t, char const *, int)" and
    then casting the return value to "unsigned char *" is suspicious.

Error: SIZEOF_MISMATCH (CWE-569):
    libvirt-0.10.2/tools/virsh-domain.c:4942: suspicious_sizeof: Passing
    argument "8UL /* sizeof (cpumap) */" to function
    "_vshCalloc(vshControl *, size_t, size_t, char const *, int)" and
    then casting the return value to "unsigned char *" is suspicious.