Use virtio-serial, remove other vmchannel methods.

This adds support for virtio-serial, and removes all other
vmchannel methods.

Virtio-serial is faster than other methods, and is now widely
available.

I tested this by using the guestfs_upload API on an 83 MB file:
  before: 6.12 seconds (14.1 MB/sec)
   after: 4.20 seconds (20.6 MB/sec)
(note this is with the current 8K chunk size)