tools/libxl: Infrastructure for reading a libxl migration v2 stream

tools/libxl: Infrastructure for reading a libxl migration v2 stream

This contains the event machinery and state machines to read an act on
a non-checkpointed migration v2 stream (with the exception of the
xc_domain_restore() handling which is spliced later in a bisectable
way).

It also contains some boilerplate to help support checkpointed
streams, which shall be introduced in a later patch.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
v4:
 * Don't make _init() mandatory
 * Don't unilaterally clobber rc from datacopier callback
 * Comment adjustments
 * More assertions

v3: More descriptions of the internal state, shuffle function order, add
    an _init() call, condense error handling