libxl: add support for 'channels'

A 'channel':
  - is a low-bandwidth private communication channel that resembles
    a physical serial port.
  - is implemented as a PV console with a well-known string name
    which is used to hook the channel to the appropriate software
    in the guest (i.e. some kind of guest agent).
  - has a backend 'connection' which describes what should happen
    to the data.

The following 'connection' types are defined:

 * PTY: the I/O surfaces as a pty in the backend domain
 * SOCKET: a listening Unix domain socket accepts a connection in
   the backend domain and proxies

Channels may be listed but don't currently support hotplug/unplug.

Signed-off-by: David Scott <dave.scott@citrix.com>
xl: move 'replace_string' further up the file

This allows the function to be reused more easily.

Signed-off-by: David Scott <dave.scott@citrix.com>
xl: add 'xstrdup' next to 'xrealloc'

Signed-off-by: David Scott <dave.scott@citrix.com>
xl: add 'trim' and 'split_string_into_pair' functions

Signed-off-by: David Scott <dave.scott@citrix.com>
xl: add support for 'channels'

This adds support for channel declarations of the form:

 channel = [ "name=...,connection=...[,path=...][,backend=...]" ]

where 'name' is a label to identify the channel to the frontend.

If 'connection = pty' then the channel is connected to a pty in the
 backend domain
If 'connection = socket' then the channel is connected to a Unix domain
 socket given by 'path = ...' in the backend domain.

This patch also adds the command:

 xl channel-list <domain>

which allows the state of channels to be queried. In particular if
'connection=pty' this will show the path of the pty slave device.

Signed-off-by: David Scott <dave.scott@citrix.com>