bochs: Use unsigned variables for offsets and sizes (CVE-2014-0147)

Gets us rid of integer overflows resulting in negative sizes which
aren't correctly checked.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
