webadmin: add memory option to create snapshot popup

If the user choose to create snapshot for running VM, the create
snapshot popup will now provide an option whether to take snapshot for
the memory as well or not (disks only) by a checkbox.

Change-Id: Iab771d2f9ae18e7812fe8ee9806a65cbb6576a21
Bug-Url: https://bugzilla.redhat.com/960931
Signed-off-by: Arik Hadas <ahadas@redhat.com>