frontend: More descriptive message on attempt to update running VM

Changed VM_CANNOT_UPDATE_ILLEGAL_FIELD message to be more
descriptive and user-friendly.

Change-Id: I61c223134bbb3674572c23fa58485e1f500fd368
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=987941
Signed-off-by: Martin Betak <mbetak@redhat.com>