qemu: Ignore missing query-migrate-parameters

qemu: Ignore missing query-migrate-parameters

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=1441934

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
(cherry picked from commit ac58c036065ff1d53bc05b796093287c27c71762)