Introduce a "xen-load-devices-state" QAPI command that can be used to
load the state of all devices, but not the RAM or the block devices of
the VM.

We only have hmp commands savevm/loadvm, and qmp commands
xen-save-devices-state.

We use this new command for COLO:
1. suspend both primary vm and secondary vm
2. sync the state
3. resume both primary vm and secondary vm

In such case, we need to update all devices' state in any time.
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>
Signed-off-by: NChanglong Xie <xiecl.fnst@cn.fujitsu.com>
Reviewed-by: NAnthony PERARD <anthony.perard@citrix.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NStefano Stabellini <sstabellini@kernel.org>