tests: Add parameter to qtest_init_without_qmp_handshake

tests: Add parameter to qtest_init_without_qmp_handshake

Allow callers to choose whether to allow OOB support during a test;
for now, all existing callers pass false, but the next patch will
add a new caller.  Also, rewrite the monitor setup to be generic
(using the -qmp shorthand is insufficient for honoring the parameter).

Based on an idea by Peter Xu, in <20180326063901.27425-8-peterx@redhat.com>

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180327013620.1644387-4-eblake@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>