migration-test: Clean up string interpolation into QMP, part 1

Leaving interpolation into JSON to qmp() is more robust than building
QMP input manually, as explained in the recent commit "tests: Clean up
string interpolation into QMP input (simple cases)".

migrate_recover() builds QMP input manually because wait_command()
can't interpolate.  Well, it can since the previous commit.  Simplify
accordingly.

Bonus: gets rid of a non-literal format string.  A step towards
compile-time format string checking without triggering
-Wformat-nonliteral.

Cc: Juan Quintela <quintela@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-19-armbru@redhat.com>