gnt-instance modify: Support new-style NIC/disk modifications

This patch adds support for adding/removing NICs/disks at arbitrary
indices on the command line. To add a disk at a specified index, use
“--disk 3:size=16G”. To remove the second disk, use “--disk 2:remove”.
Unittests are included.

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