qemu_capabilities: Probe SEV capabilities even for QEMU_CAPS_SEV_SNP_GUEST

qemu_capabilities: Probe SEV capabilities even for QEMU_CAPS_SEV_SNP_GUEST

While it's very unlikely to have QEMU that supports SEV-SNP but
doesn't support plain SEV, for completeness sake we ought to
query SEV capabilities if QEMU supports either. And similarly to
QEMU_CAPS_SEV_GUEST we need to clear the capability if talking to
QEMU proves SEV is not really supported.

This in turn removes the 'sev-snp-guest' capability from one of
our test cases as Peter's machine he uses to refresh capabilities
is not SEV capable. But that's okay. It's consistent with
'sev-guest' capability.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>