libxl-ev-qmp-staging

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

Changes in v5:
    Plenty of patch have been applied.
    Other changes mostly are coding style and other typos.
    Some bug fixes.
    Details can be found in patch notes.

    I have left aside the change to cdrom_insert until I can found what to do
    with the userdata lock.

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

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.

Patches series available in a git tag:
git fetch https://xenbits.xen.org/git-http/people/aperard/xen-unstable.git libxl-ev-qmp-v5
git checkout -b libxl-ev-qmp-v5 FETCH_HEAD

Cheers,