bochs: Check catalog_size header field (CVE-2014-0143)

It should neither become negative nor allow unbounded memory
allocations. This fixes aborts in g_malloc() and an s->catalog_bitmap
buffer overflow on big endian hosts.

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>
