KVM: selftests: Skip AMX test if ARCH_REQ_XCOMP_GUEST_PERM isn't supported

Skip the AMX test instead of silently returning if the host kernel
doesn't support ARCH_REQ_XCOMP_GUEST_PERM.  KVM didn't support XFD until
v5.17, so it's extremely unlikely allowing the test to run on a pre-v5.15
kernel is the right thing to do.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-40-seanjc@google.com