Migration with old QEMU which does not support query-migrate-parameters
would fail because the QMP command is called unconditionally since the
introduction of TLS migration. Previously it was only called if the user
explicitly requested a feature which uses QEMU migration parameters. And
even then the situation was not ideal, instead of reporting an
unsupported feature we'd just complain about missing QMP command.

Trivially no migration parameters are supported when
query-migrate-parameters QMP command is missing. There's no need to
report an error if it is missing, the callers will report better error
if needed.

https://bugzilla.redhat.com/show_bug.cgi?id=1441934Signed-off-by: NJiri Denemark <jdenemar@redhat.com>