qemu: put vhost-user code that's special for passt in a helper function

qemu: put vhost-user code that's special for passt in a helper function

Rather than duplicating these two lines of chr device object setup for
hotplug and domain start, put them in a helper function that's called
from both places. That way when we need to setup *more* stuff specific
to passt+vhostuser, we can just add it in that one place.

Signed-off-by: Laine Stump <laine@redhat.com>
Tested-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>