Eliminate --disable-gfx-check and make VNC default when SDL not available

Eliminate --disable-gfx-check and make VNC default when SDL not available

 --disable-gfx-check predates VNC server support.  It made sense back then
because the only thing you could do without SDL was use -nographic mode or
similar tricks.  Since this is a very advanced mode of operation, gfx-check
provided a good safety net for casual users.

A casual user is very likely to use VNC to interact with a guest.  In fact, it's
often frustrating to install QEMU on a server and have to specify
disable-gfx-check when you only want to use VNC.

This patch eliminates disable-gfx-check and makes SDL behave like every other
optional dependency.  If SDL is not available, instead of failing ungracefully
if no special options are specified, we default to -vnc localhost:0,to=99.
When we do default to VNC, we also print a message to tell the user that we've
done this include which port we're currently listening on.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>