qemu: Reject SDL graphic if it's not supported by qemu

qemu: Reject SDL graphic if it's not supported by qemu

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