Add virtio-console support to bhyve.

Adds virtio-console device support to bhyve, allowing to create
bidirectional character streams between host and guest.

Syntax:
-s <slotnum>,virtio-console,port1=/path/to/port1.sock,anotherport=...

Maximum of 16 ports per device can be created. Every port is named
and corresponds to an Unix domain socket created by bhyve. bhyve
accepts at most one connection per port at a time.

Limitations:

• due to lack of destructors of in bhyve, sockets on the filesystem must be cleaned up manually after bhyve exits
• there's no way to use "console port" feature, nor the console port resize as of now
• emergency write is advertised, but no-op as of now

Approved by: trasz
MFC after: 1 month
Relnotes: yes
Sponsored by: iXsystems, Inc.
Differential Revision: D7185