webadmin: preview snapshot with memory popup

New popup is now shown when the user choose to preview snapshot that has
memory. the user can select not to use the saved memory when previewing
the snapshot from this popup.

Change-Id: I18c99b22b33256e6bcf1cb9b1151e908e27ded8d
Bug-Url: https://bugzilla.redhat.com/960931
Signed-off-by: Arik Hadas <ahadas@redhat.com>