virtualx.eclass: Fix no display for an emerge following a failure
If using GNOME GDM, X is started on DISPLAY :0 but a lock file
/tmp/.X1024-lock is created instead of /tmp/.X0-lock.
virtx() will initially set XDISPLAY to 0 and attempt to start Xvfb on
DISPLAY :0 which fails but DISPLAY :1 (and greater) is not attempted if
a previous emerge left /tmp/.X1-lock behind.
Closes: https://bugs.gentoo.org/690778
Signed-off-by: Chris Mayo <aklhfex@gmail.com>
Signed-off-by: Matt Turner <mattst88@gentoo.org>