hvf: Guard xgetbv call

This prevents illegal instruction on cpus that do not support xgetbv.

Buglink: https://bugs.launchpad.net/qemu/+bug/1758819


Reviewed-by: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Hill Ma <maahiuzeon@gmail.com>
Message-Id: <X/6OJ7qk0W6bHkHQ@Hills-Mac-Pro.local>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>