Merge remote-tracking branch 'remotes/elmarco/tags/dbus-vmstate7-pull-request' into staging

Add dbus-vmstate

Hi,

With external processes or helpers participating to the VM support, it
becomes necessary to handle their migration. Various options exist to
transfer their state:
1) as the VM memory, RAM or devices (we could say that's how
   vhost-user devices can be handled today, they are expected to
   restore from ring state)
2) other "vmstate" (as with TPM emulator state blobs)
3) left to be handled by management layer

1) is not practical, since an external processes may legitimatelly
need arbitrary state date to back a device or a service, or may not
even have an associated device.

2) needs ad-hoc code for each helper, but is simple and working

3) is complicated for management layer, QEMU has the migration timing

The proposed "dbus-vmstate" object will connect to a given D-Bus
address, and save/load from org.qemu.VMState1 owners on migration.

Thus helpers can easily have their state migrated with QEMU, without
implementing ad-hoc support (such as done for TPM emulation)

D-Bus is ubiquitous on Linux (it is systemd IPC), and can be made to
work on various other OSes. There are several implementations and good
bindings for various languages.  (the tests/dbus-vmstate-test.c is a
good example of how simple the implementation of services can be, even
in C)

dbus-vmstate is put into use by the libvirt series "[PATCH 00/23] Use
a slirp helper process".

v2:
 - fix build with broken mingw-glib

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# gpg: Good signature from "Marc-André Lureau <marcandre.lureau@redhat.com>" [full]
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* remotes/elmarco/tags/dbus-vmstate7-pull-request:
  tests: add dbus-vmstate-test
  tests: add migration-helpers unit
  dockerfiles: add dbus-daemon to some of latest distributions
  configure: add GDBUS_CODEGEN
  Add dbus-vmstate object
  util: add dbus helper unit
  docs: start a document to describe D-Bus usage
  vmstate: replace DeviceState with VMStateIf
  vmstate: add qom interface to get id

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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