tools/libxl: Infrastructure for writing a v2 stream

tools/libxl: Infrastructure for writing a v2 stream

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

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, use FILLZERO(), adjust stream_success()
v3: Similar changes to the read side, following the read side review.