core: change shared-lock msg of AddVmFromTemplate

This patch adds a custom error message for the scenario in which a
command fails to run because of the shared-lock that
AddVmFromTemplate command acquires on its given template.

The new error message says that the command cannot be executed because
the template is used for creation of VM, and shows the name of that VM.

Change-Id: I9fcc4d70bd5ca28c87ca7834bf2b699da5140fc5
Signed-off-by: Arik Hadas <ahadas@redhat.com>