uio_hv_generic: defer opening vmbus until first use

This fixes two design flaws in hv_uio_generic.

Since hv_uio_probe is called from vmbus_probe with lock held
it potentially can cause sleep in an atomic section because
vmbus_open will wait for response from host.

The hv_uio_generic driver could not handle applications
exiting and restarting because the vmbus channel was
persistent.  Change the semantics so that the buffers are
allocated on probe, but not attached to host until
device is opened.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>