uio_hv_generic: set callbacks on open

This fixes the problem where uio application was unable to
use multple queues on restart. The root cause is that the callbacks
are cleared on disconnect. Change to setting up callbacks
everytime in open.

Fixes: cdfa835c ("uio_hv_generic: defer opening vmbus until first use")
Reported-by: Mohammed Gamal <mgamal@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>