libvirt: Supoort libvirt pool xml accessor

Signed-off-by: Yanbing Du <ydu@redhat.com>
libvirt: Add test for virsh pool-create command

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #952 from will-Do/virsh_pool_branch

libvirt: Supoort libvirt pool xml accessor and add a test for pool-create