[libvirt] storage:lvm pool creation fails because of vg not built

[lvroyce]

1.how to reproduce
the reproduce steps just as 
http://www.redhat.com/archives/libvir-list/2011-November/msg01151.html

2.analyse:
since create = define + start
so pool-create can't work for the vg pre-built condition,
shall we change create = define + build + start(if vg does not exist)
and create=define+start(if vg exists)?

Thank you for your suggestion!