virtio-blk: Allow extended partitions

This will allow up to DISK_MAX_PARTS (256) partitions, with for example
GPT in the guest. Otherwise, the partition scan code will only discover
the first 15 partitions.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>