migration-test: Make wait_command() cope with '%'

wait_command() passes its argument @command to qtest_qmp_send().
Falls apart if @command contain '%'.  Two ways to disarm this trap:
suppress interpretation of '%' by passing @command as argument to
format string "%s", or fix it by having wait_command() take the
variable arguments to go with @command.  Do the latter.

This is another 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: <20180806065344.7103-18-armbru@redhat.com>