Unbreak -no-quit for GTK, validate SDL options

Certain options (-no-frame, -alt-grab, -ctrl-grab) only make sense with SDL.
When compiling without SDL, these options (and -no-quit) print an error message
and exit qemu.

In case QEMU is compiled with SDL support, the three aforementioned options
still do not make sense with other display types. This patch addresses that
issue by printing a warning. I have chosen not to exit QEMU afterwards because
having the option is not harmful and before this patch it would be ignored
anyway.

By delaying the sanity check from compile-time with some ifdefs to run-time,
-no-quit is now also properly supported when compiling without SDL.

Signed-off-by: Peter Wu <lekensteyn@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
