If the emulator doesn't support SDL graphic, we should reject
the use of SDL graphic xml with error messages, but not ignore
it silently, and pretend things are fine.

"-sdl" flag was exposed explicitly by qemu since 0.10.0, more detail:
http://www.redhat.com/archives/libvir-list/2011-January/msg00442.html

And we already have capability flag "QEMUD_CMD_FLAG_0_10", which
could be used to prevent the patch affecting the older versions
of QEMU.

* src/qemu/qemu_command.c