webadmin: preview partial snapshot confirmation dialog

When selecting to preview a snapshot that contains only
a subset of the VM disks, a 'Preview Partial Snapshot'
dialog should be displayed. The dialog suggests a couple
of options to the user for proceeding (see screenshot).

Screenshot: http://i.imgur.com/m7wsgB8.png

Change-Id: Iabd59025d7c7b96ae92898efe34e20985d2e9750
Bug-Url: https://bugzilla.redhat.com/1095808
Signed-off-by: Daniel Erez <derez@redhat.com>