usb-hid: activate usb tablet / mouse after migration.

usb-hid: activate usb tablet / mouse after migration.

qemu uses the ps/2 mouse by default.  The usb tablet (or mouse) is
activated as soon as qemu sees some guest activity on the device,
i.e. polling for HID events.  That used to work fine for both fresh
boot and migration.

Remote wakeup support changed the picture though: There will be no
polling after migration in case the guest suspended the usb bus,
waiting for wakeup events.  Result is that the ps/2 mouse stays
active.

Fix this by activating the usb tablet / mouse in post_load() in case
the guest enabled remote wakeup.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>