Helper processes may have their state migrated with QEMU data stream
thanks to the QEMU "dbus-vmstate".

libvirt maintains the list of helpers to be migrated. The
"dbus-vmstate" is added when required, and given the list of helper
Ids that must be migrated, on save & load sides.

See also:
https://git.qemu.org/?p=qemu.git;a=blob;f=docs/interop/dbus-vmstate.rstSigned-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>