webadmin: add 'import' functionality in vms tab

Add 'import' button to the Virtual Machines tab that opens a dialog from
which we can import VMs from different sources into the system.

For now, this dialog allows to import VMs only from the export domain.

Change-Id: I9aaae633518b1268e63b36894ba9e077884d4694
Signed-off-by: Arik Hadas <ahadas@redhat.com>