libqtest: rename qmp() to qmp_discard_response()

libqtest: rename qmp() to qmp_discard_response()

Existing qmp() callers do not expect a response object.  In order to
implement real QMP test cases it will be necessary to inspect the
response object.

Rename qmp() to qmp_discard_response().  Later patches will introduce a
qmp() function that returns the response object and tests that use it.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>