qxl: error handling fixes and cleanups.

Add qxl_guest_bug() function which is supposed to be called in case
sanity checks of guest requests fail.  It raises an error IRQ and
logs a message in case guest debugging is enabled.

Make PANIC_ON() abort instead of exit.  That macro should be used
for qemu bugs only, any guest-triggerable stuff should use the new
qxl_guest_bug() function instead.

Convert a few easy cases from PANIC_ON() to qxl_guest_bug() to
show intended usage.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
