Allow param `modify' during gnt-instance modify

With the new gnt-instance modify syntax for addition and removal of
disks/NICs on arbitrary indexes, we hit an assertion if the user
passes `modify' as one of the disk's parameters. E.g::

 gnt-instance modify --disk 2:modify,size=3G instance1
 gnt-instance modify --disk 3:add,size=1G,modify instance2

This patch fixes the bug, by allowing `modify' to be passed as a
parameter (as happens with `add' and `remove'), as long as it is
not done alongside `add' or `remove'. If so, it is treated in the
same way as if none of modify/add/remove is passed --> modify.

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>