tests/libqtest.c: Delete possible stale unix sockets

Occasionally if a test crashes or is interrupted by the user
at the wrong moment it could leave behind a stale UNIX
socket in /tmp/. This will then cause a subsequent test
run to fail spuriously with
 tests/libqtest.c:70:init_socket: assertion failed (ret != -1): (-1 != -1)
if it happens to reuse the same PID.

Defend against this by deleting any stray stale socket before
trying to open the new ones for this test.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1490963801-27870-1-git-send-email-peter.maydell@linaro.org
