virDomainVcpuPinDefFree() does not free def->cputune.vcpupin if nvcpupin
is 0, and does not set def->cputune.vcpupin to NULL.
If we set nvcpupin to 0 but do not free vcpupin, vcpupin will not be freed
when vm->def is freed.
Use VIR_FREE() instead of virDomainVcpuPinDefFree() to free the memory
and set def->cputune.vcpupint to NULL.