usb-host: scan for usb devices when the vm starts

usb-host: scan for usb devices when the vm starts

Commit a844ed842d9a9d929645c09ae0f52f753d7a02e0 leads to usb-host
detecting devices not right after qemu startup because the guest
isn't running yet.  Instead they are found on the first of the
regular usb device poll runs.  Which is too late for seabios to see
them, so booting from usb sticks fails.

Fix this by adding a vm state change handler which triggers a device
scan when the vm is started.

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