virHostCPUGetCPUID: Add comment on how KVM_GET_SUPPORTED_CPUID works
virHostCPUGetCPUID: Add comment on how KVM_GET_SUPPORTED_CPUID works

The commit adding the code fetching host CPU flags via the
KVM_GET_SUPPORTED_CPUID didn't describe at all why such an alghorithm is
needed.

Add a comment from the documentation outlining how the userspace
function is expected to allocate memory here.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Jiri Denemark <62bdf77dc47919a4d59a91822129d14633cfca81@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,crobinso/libvirt