virttest.virsh: Add virsh vol-create command wrapper

Signed-off-by: Wayne Sun <gsun@redhat.com>
virttest.libvirt_xml: Add new accessors and function in vol_xml module

Add new accessors and attributes for volume xml operation.
Add descrtiption for new add properties and fix some exist ones.
Add new function new_vol to get new VolXML instance with given params.
Import vol_xml module class in package init file.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1545 from waynesun09/volume_operation

Update vol_xml and add vol-create command wrapper