Currently the TLS session object assumes that the caller will always
provide a hostname when using x509 creds on a client endpoint. This
relies on the caller to detect and report an error if the user has
configured QEMU with x509 credentials on a UNIX socket. The migration
code has such a check, but it is too broad, reporting an error when
the user has configured QEMU with PSK credentials on a UNIX socket,
where hostnames are irrelevant.

Putting the check into the TLS session object credentials validation
code ensures we report errors in only the scenario that matters.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220304193610.3293146-2-berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>