vga: fix display update region calculation (split screen)

vga display update mis-calculated the region for the dirty bitmap
snapshot in case split screen mode is used.  This can trigger an
assert in cpu_physical_memory_snapshot_get_dirty().

Impact:  DoS for privileged guest users.

Fixes: CVE-2017-13673
Fixes: fec5e8c9
Cc: P J P <ppandit@redhat.com>
Reported-by: David Buchanan <d@vidbuchanan.co.uk>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170828123307.15392-1-kraxel@redhat.com