bhyve: TPM 2.0 emulation with swtpm

Implement a TPM 2.0 emulation backend to connect to a running swtpm
instance using a UNIX domain socket.

Reviewed by: corvink
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46373

(cherry picked from commit d93fbcf0222b05fcd7704b35bc94e5513c2682e2)