webadmin: Allow reboot without Guest Agent

Added check to enable reboot command when acpi is enabled even when
guest agent is not present.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1153711
Change-Id: I7e546122d27786465f355c112b218b128b357d1a
Signed-off-by: Martin Betak <mbetak@redhat.com>