webadmin: Multiple consoles in WebAdmin

Multiple consoles can be opened for selected VMs in WebAdmin
using one command.

The Console button/menu item is enabled, when at least one VM has
a valid console.

Tested for Spice plugin as well as MIME attachment servlet.

Change-Id: I894eaeafab22614af3f219169ef73d7fbeed3a11
Signed-off-by: Libor Spevak <lspevak@redhat.com>