engine: Display warning before overtaking spice console

This patch adds a displaying popup window when an user is overtaking (Spice or
VNC) console that has been already in use.

The popup window is displayed if these conditions hold:
 - "allow console reconnect" feature is disabled
 - there is a user currently connected to the console and following conditions
   hold:
	 - the user currently connected to the console is different from the user
           who is trying to connect
	 - the user currently connected to the console has not permissions to
           reconnect (steal) console

Changes on backend:
 - Added command for checking whether an ad element has permission that allows
   console reconnect.
 - Removed UpdateVmConsoleData command and parameters. This command served only
   for unsetting console user on console disconnect. This is done by
   VdsEventListener.
 - Removed console related fields from VdsUpdateRuntimeInfo.

Changes on frontend:
 - Added methods for checking safeness of console connecting operation and
   displaying confirm popup.

Change-Id: Iffce89a0c7bdd0cccb71943f398f0f907fbed002
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=868297