qxl: factor out qxl_get_check_slot_offset

New helper function which translates a qxl physical address into
memory slot and offset.  Also applies sanity checks.  Factored out
from qxl_phys2virt.  No functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1466597244-5938-1-git-send-email-kraxel@redhat.com
