core: support for partial snapshot creation

Introducing support for creating a snapshot using
a specified disks list, i.e. taking a snapshot only
on the selected disks.

* CreateAllSnapshotsFromVmParameters:
** Added an optional disks list member.

* CreateAllSnapshotsFromVmCommand:
** Updated accordingly to handle the specified disks list.

* SnapshotManager:
** Added addActiveSnapshot method signature that accepts
   disks list argument.
** Modified addSnaphot methods accordingly.

Feature: http://www.ovirt.org/Features/Single_Disk_Snapshot

Change-Id: Ieb7cb389229219bbb0212e3ee58cc45c39727c75
Signed-off-by: Daniel Erez <derez@redhat.com>