dbus-display: fix test race when initializing p2p connection

dbus-display: fix test race when initializing p2p connection

The D-Bus connection starts processing messages before QEMU has the time
to set the object manager server. This is causing dbus-display-test to
fail randomly with:

ERROR:../tests/qtest/dbus-display-test.c:68:test_dbus_display_vm:
assertion failed
(qemu_dbus_display1_vm_get_name(QEMU_DBUS_DISPLAY1_VM(vm)) ==
"dbus-test"): (NULL == "dbus-test") ERROR

Use the delayed message processing flag and method to avoid that
situation.

(the bus connection doesn't need a fix, as the initialization is done
synchronously)

Reported-by: Robinson, Cole <crobinso@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Cole Robinson <crobinso@redhat.com>
Message-Id: <20220609152647.870373-1-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>