xen/pvcalls: implement read

xen/pvcalls: implement read

When an active socket has data available, increment the io and read
counters, and schedule the ioworker.

Implement the read function by reading from the socket, writing the data
to the data ring.

Set in_error on error.

Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
CC: [email protected]
CC: [email protected]
Signed-off-by: Boris Ostrovsky <[email protected]>