Check for None when cleaning PCI dev usage

The PCI manager needs to check for instance UUIDs mapped to
None when cleaning up it's internal claims/allocations maps.

Also adds a check so the claims dict doesn't map to None in
the first place.  There was already a check like this when
setting a value in the allocations dict.

Closes-Bug: #1238374

Change-Id: I7ac010f7d50ad85c145e536cfd48e07c8f4602d3
(cherry picked from commit c28c87db4c2b20e0f6c1375d2d4433e06c4e1743)
Merge "Check for None when cleaning PCI dev usage" into stable/havana