1156576: Fix NPE on hypervisor checkin

The NPE was occurring if a request was made with an empty body.

This was fixed by checking for a null host to guest mapping
and throwing a BadRequestException. I had considered
equating the null data as an empty mapping and allowing
the request to succeed, but IMO the request should be
made with at least an empty mapping.
Merge pull request #797 from candlepin/mstead/fix-npe-on-hypervisor-check-in

1156576: Fix NPE on hypervisor checkin