vhost+postcopy: Helper to send requests to source for shared pages

vhost+postcopy: Helper to send requests to source for shared pages

Provide a helper to be used by shared waker functions to request
shared pages from the source.
The last_rb pointer is moved into the incoming state since this
helper can update it as well as the main fault thread function.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>