virHostCPUGetCPUID: Fix possible allocation of huge amount of memory

virHostCPUGetCPUID: Fix possible allocation of huge amount of memory

In case when the 'KVM_GET_SUPPORTED_CPUID' ioctl on /dev/kvm would
fail for other reason than the documented E2BIG, our code would continue
looping and calling it while always increasing the memory buffer even
when that will not help.

Rewrite the function to allow another iteration only with the correct
errno.

Additionally rename the 'i' variable to 'alloc_size' as it's not a pure
iterator.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>