engine: Edit disk description when VM is up

Since the description is just a database field, there's no (good)
reason to prevent editing disks' descriptions while the related VM(s)
is(are) up.

This patch includes:
- Fixing the UpdateVmDiskCommand to allow such updates
- Adding cases to UpdateVmDiskCommandTest to check this functionality
- Fixing EditDiskModel to allow this update in the UI

Change-Id: Id116aec43abb8863faf17a240530dac0a78f22e0
Signed-off-by: Allon Mureinik <amureini@redhat.com>