Fixing crash when removing disks

This bug was introduced by my commit "Check ispecs against ipolicy on
instance modify", because I did not filter for the 'add' action. Thus it
would try to read the size parameter also when removing disks.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>