Add test for virsh domblkinfo v4

According to Chris and yu's sugguestion, I modified some places.
And I resend this pullreq with commit messages from my own domblkinfo branch.

Li Yang (3):
  libvirt: Add test module for virsh domblkinfo
  virt: Add virsh function for virsh domblkinfo
  virt: Add get_disk_source() for virsh domblkinfo

 libvirt/tests/cfg/virsh_domblkinfo.cfg |   47 +++++++++++
 libvirt/tests/virsh_domblkinfo.py      |  132 ++++++++++++++++++++++++++++++++
 virttest/libvirt_xml/vm_xml.py         |   16 +++-
 virttest/virsh.py                      |   12 +++
 4 files changed, 204 insertions(+), 3 deletions(-)
 create mode 100644 libvirt/tests/cfg/virsh_domblkinfo.cfg
 create mode 100644 libvirt/tests/virsh_domblkinfo.py