Added "vg" argument to the --disk syntax

Added "vg" argument to the "gnt-instance ... --disk" syntax specification.
Now is ok to write:

gnt-instance add ... --disk N:size=NNg,vg=VG_NAME ...

But not all internal structures are ready to handle this yet.

Signed-off-by: Dmitry Chernyak <dmi.chernyak@gmail.com>
[iustin@google.com: removed changes to design-2.0.rst]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>