libxl-migration-fdset-staging

libxl: Enable save/restore/migration of a restricted QEMU + libxl__ev_qmp_*

In order for libxl to be able to manage QEMU while it is restricted, a few
changes are needed. We need a new way to get a startup notification from QEMU
as xenstore may not be accessible when QEMU is ready. We also need to a
different way to have QEMU save it's state and to insert cdrom as a restricted
QEMU doesn't have access to the file system.

For both, we can use QMP, we can use it to query QEMU's status, and we can use
it to send a file descriptor through which QEMU can save its state, or it can
be a cdrom.

We take this opportunity to rewrite the QMP client, and this time been
asynchronous, the result is libxl__ev_qmp_*.

The plat de résistance in this patch series start with patch
"libxl: Design of an async API to issue QMP commands to QEMU"
which implement libxl__ev_qmp_* functions to turn the QMP client into
asynchronous mode.

This comes with changes that uses the new interface.
* "libxl: QEMU startup sync based on QMP"
  which can use QMP to find out when QEMU as started.
  this requires: "libxl_dm: Pre-open QMP socket for QEMU"
  But that only works with dm_restrict=1 as explain in the patch.
* "libxl: Re-implement domain_suspend_device_model using libxl__ev_qmp"
  Which rewrite libxl__qmp_save(), and adds the ability to have QEMU save
  its state to a file descriptor which libxl will have openned.
* "libxl_disk: Have libxl_cdrom_insert use libxl__ev_qmp"
  Which rewrites libxl__qmp_insert_cdrom() and adds the ability for libxl
  to open the cdrom on behalf of QEMU.

The first few patches do some cleanup and fixes of the current qmp client
implementation, mostly because it bothered me as I think we should remove the
current implementation. They could be commited ahead of libxl__ev_qmp.

Changes in v4:
    Better API which meant a lot of other changes.

Patches series available in a git tag:
git fetch https://xenbits.xen.org/git-http/people/aperard/xen-unstable.git libxl-migration-fdset-v4
git checkout -b libxl-migration-fdset-v4 FETCH_HEAD

Cheers,