Most functions which handle exceptions don't return.

With a compiler attribute (added by QEMU_NORETURN),
gcc can optimize the code.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>