bhyve: emulate TPM passthru by accessing /dev/tpmX

To send commands to the TPM device, bhyve can use the host TPM driver by
reading and writing from /dev/tpmX. Using this approach, only the host
TPM driver has to detect and interact with the physical TPM interface.
This simplifies bhyve's code much. As the host TPM driver has to
interact with the TPM regardless of bhyve making use of it or not, makes
it a good approach.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40460