Fix bug in “gnt-node list-storage”

LVM PV storage units would always show as allocatable, even when they
weren't. For some reason I have not been able to determine, the function
parsing the attributes (“_GetAllocatable”) was not even called and the
list opcode simply returned the attribute string as the value (e.g.
“a-”).  Removing “@staticmethod” did the trick and then I just moved it
to module level.

A QA test is included.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>