block: Fix serial number assignment

We should use 'dinfo->serial' length, 'serial' is a pointer, so
the serial number length is currently limited to the pointer size.

This fixes https://bugs.launchpad.net/qemu/+bug/584143 and is also
valid for stable.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
