qtest: Use -display none by default

This avoids each test needing to add it to suppress windows popping up.

[Commit 7ceeedd016facf8d58e14a0d1417fa7225d71072 ("blockdev-test: add
test case for drive_add duplicate IDs") and commit
43cd209803d6cffb1e1a028c9ff2fd0ff4fce954 ("qdev-monitor-test: add
device_add leak test cases") added qtest tests without specifying
-display none.

As a result, "make check" now tries to use graphics (GTK or SDL).  Since
graphics are not used by the test and inappropriate for headless "make
check" runs, add the missing -display none.

This fixes "make check" in the QEMU buildbot.
-- Stefan]

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