core: change import vm to be VmCommand - part 1

This patch reduces the coupling between ImportVmCommand and
MoveOrCopyTemplateCommand command by replacing calls to methods
that always return the same constants with the constant values.

Change-Id: I77c044c25246ebd2fb6e5b9999bfbe2267ab3cbe
Signed-off-by: Arik Hadas <ahadas@redhat.com>