qxl: Ignore set_client_capabilities pre/post migrate

The recent introduction of set_client_capabilities has broken
(seamless) migration by trying to call qxl_send_events pre (seamless
incoming) and post (*) migration, triggering the following assert:
qxl_send_events: Assertion `qemu_spice_display_is_running(&d->ssd)' failed.

The solution is easy, pre migration the guest will have already received
the client caps on the migration source side, and post migration there no
longer is a guest, so we can simply ignore the set_client_capabilities call
in both those scenarios.

*) Post migration, so not fatal for to the migration itself, but still a crash

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
