webadmin: replace 'VM' with 'Virtual Machine'

Replace 'VM' term with 'Virtual Machine' in newly added string that was
added to the dialog which is shown when trying to import an already
exist vm.

Change-Id: I126e247a2471b9bdb25d927700427ed51f3d3a34
Bug-Url: https://bugzilla.redhat.com/878495
Signed-off-by: Arik Hadas <ahadas@redhat.com>