Make rpc.call_lv_list() VG-aware

Changes to backend.GetVolumeList():
- now accepts a list of VGs instead of one VG
- returns LV names in the form "vg_name/lv_name"

Corresponding changes are done in: VerifyDisks, VerifyNode,
LUCreateInstance (for both disk creation and adoption cases)

Now the syntax
"gnt-instance add ... --disk N:adopt=LV_NAME,vg=VG_NAME"
as was described earlier in the man page works.

Signed-off-by: Dmitry Chernyak <dmi.chernyak@gmail.com>
[iustin@google.com: QA changes for reserved LVs, style fixes and a few
 extra error checks, reviewed by hansmi/rn]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>