qemu: ignore query-sev-capabilities errors

qemu: ignore query-sev-capabilities errors

The query-sev-capabilities command fails if SEV is not compiled in,
even though both the command and -object sev-guest are present
in that case :/

Ignore the errors to avoid spamming the logs:
internal error: unable to execute QEMU command 'query-sev-capabilities': SEV feature is not available

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>