libqos: Use explicit QTestState for remaining libqos operations

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>