</disk><disk type="block" device="disk">
  <driver name="qemu" type="raw"/>
  <source dev="/dev/tank/foobar"/>
  <backingStore/>
  <target dev="vdb" bus="virtio"/>
  <alias name="virtio-disk1"/></disk>

# lvs
  foobar                                      tank           -wi-ao----   4.00g

# lvresize -L +2G /dev/tank/foobar

# virsh blockresize myguest /dev/tank/foobar --size 0

guest# resize2fs 1.42.12 (29-Aug-2014)
Resizing the filesystem on /dev/vdb to 1572864 (4k) blocks.
The filesystem on /dev/vdb is now 1572864 (4k) blocks long.