tests/qtest: get rid of 'qmp_command' helper in migration test

tests/qtest: get rid of 'qmp_command' helper in migration test

This function duplicates logic of qtest_qmp_assert_success_ref.
The qtest_qmp_assert_success_ref method has better diagnostics
on failure because it prints the entire QMP response, instead
of just asserting on existance of the 'error' key.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230601161347.1803440-4-berrange@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>