Pull the check for VM ownership into ppt_find().

This reduces some code duplication. One behavior change is that
ppt_assign_device() will now only succeed if the device is unowned.
Previously, a device could be assigned to the same VM multiple times,
but each time it was assigned, the device's state was reset.

Reviewed by: markj, grehan
MFC after: 2 weeks
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D27301