libvirt.tests: Add virsh vol-create command case

Add test case for virsh command vol-create, prepare different type
of pools before run the command within the case.

The case covers pool of default format with supported volume format
matrix.

Reference:
http://www.libvirt.org/storage.html

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #70 from waynesun09/vol_create

libvirt.tests: Add virsh vol-create command case