virtio-blk: Call revalidate_disk() upon online disk resize
If a virtio disk is open in guest and a disk resize operation is done,
(virsh blockresize), new size is not visible to tools like "fdisk -l".
This seems to be happening as we update only part->nr_sects and not
bdev->bd_inode size.
Call revalidate_disk() which should take care of it. I tested growing disk
size of already open disk and it works for me.