snapshot: add virsh domblklist command

snapshot: add virsh domblklist command

This adds a convenience function to virsh that parses out block
information from the domain xml, making it much easier to see
what strings can be used in all other contexts that demand a
specific block name, especially when given the previous patch
that allows using either target or unique source name.

As an example on a domain with one disk and an empty cdrom drive:

Target     Source
-------------------------------------------
vda        /var/lib/libvirt/images/fedora_12.img
hdc        -

* tools/virsh.c (cmdDomblklist): New function.
* tools/virsh.pod (domblklist): Document it.