bhyve: add virtio-console support

bhyve: add virtio-console support

Bhyve supports virtio-console devices using the following syntax:

 -s 2:0,virtio-console,org.qemu.guest_agent.0=/path/to/unix/socket,other.port=/other/socket,...

There are two details about that to consider.

The first one is that only up to 16 ports per console is supported. This
is different from the default (31), so update the code to manually add
the virtio-serial controllers with 16 ports. For the existing
controllers, make sure to set max ports to 16 or error out if ports
count greater than 16 was specified.

The second one is that bhyve does not clean up UNIX sockets for these
devices. So update virBhyveProcessStop() to remove leftover sockets.

Not adding capabilities probing as the virtio-console device is
available on all supported FreeBSD versions and on all supported arches.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>