runqemu: Allow auto detection of the correct graphics options

runqemu: Allow auto detection of the correct graphics options

Running "runqemu qemux86 kvm" when qemu is configured for sdl and/or gtk
display output currently leads to a poor user experience with no cursor
and corrupted fonts in the gtk case. This is due to no options being
passed to qemu which leads to the loss of the font envirornment variable
and the show-cursor option.

If the user hasn't specified a display type, grep the output of
"qemu-system-xxx --help" for the display types and pick the "best"
which ensures our config is passed in. That resolves the gtk font issue
and the cursor issue with both sdl and gtk.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>