The target has not seen the guest_connected event via
spice_chr_guest_open or spice_chr_write, and so spice server wrongly
assumes there is no agent active, while the client continues to send
motion events only by the agent channel, which the server ignores. The
net effect is that the mouse is static in the guest.
By registering the interface on post load spice server will pass on the
agent messages fixing the mouse behavior after migration.
Note that qemu_be_is_fe_connected is called when the vm is already
running, to avoid any possible order of vmstate loading issue, i.e.
device loading after chardev post_load being called back.