Add experimental warning for Cells

When we merged Cells a while back, we noted that the usage was
experimental but haven't had a good channel for communicating the
continued state to the user. Since we're adding warning messages
to virt drivers, this patch adds one for Cells as well, in the
Manager's init method.

Change-Id: I75b0bedf57250e7701b00f9ce0f60e4775eacfd7
Merge "Add experimental warning for Cells"