qcow2: Fix backing file name length check

qcow2: Fix backing file name length check

len could become negative and would pass the check then. Nothing bad
happened because bdrv_pread() happens to return an error for negative
length values, but make variables for sizes unsigned anyway.

This patch also changes the behaviour to error out on invalid lengths
instead of silently truncating it to 1023.

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 6d33e8e7dc9d40ea105feed4b39caa3e641569e8)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>