libqos: Use explicit QTestState for remaining libqos operations

Drop one more client of global_qtest by teaching all remaining
libqos stragglers to pass in an explicit QTestState.  Change the
setting of global_qtest from being implicit in libqos' call to
qtest_start() to instead be explicit in all clients that are
still relying on global_qtest.

Note that qmp_execute() can be greatly simplified in the process,
and that we also get rid of interpolation of a JSON string into a
temporary variable when qtest_qmp() can do it more reliably.

Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Greg Kurz <groug@kaod.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/rtas-test.c

@@ -14,6 +14,7 @@ static void test_rtas_get_time_of_day(void)
     time_t t1, t2;
 
     qs = qtest_spapr_boot("-machine pseries");
+    global_qtest = qs->qts;
 
     t1 = time(NULL);
     ret = qrtas_get_time_of_day(qs->qts, qs->alloc, &tm, &ns);