qcow2: Check refcount table size (CVE-2014-0144)

qcow2: Check refcount table size (CVE-2014-0144)

Limit the in-memory reference count table size to 8 MB, it's enough in
practice. This fixes an unbounded allocation as well as a buffer
overflow in qcow2_refcount_init().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 5dab2faddc8eaa1fb1abdbe2f502001fc13a1b21)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>