webadmin: enhance warning dialog on host restart

When restarting a host using power management, running VMs will be
ungracefully stopped. This patch adds the number of VMs on each host to
the warning dialog.

Change-Id: If1c09b529d445ce4e6c3131d227ceffb22cd292d
Signed-off-by: Oved Ourfali <oourfali@redhat.com>