selftests: KVM: allow sev_migrate_tests on machines without SEV-ES

I managed to get hold of a machine that has SEV but not SEV-ES, and
sev_migrate_tests fails because sev_vm_create(true) returns ENOTTY.
Fix this, and while at it also return KSFT_SKIP on machines that do
not have SEV at all, instead of returning 0.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>