libvchan: Make raw_get_{data_ready, buffer_space} match

libvchan: Make raw_get_{data_ready, buffer_space} match

For writing into a vchan, raw_get_buffer_space used >, allowing the full
ring size to be written.  On the read side, raw_get_data_ready compared
the ring size with >=.  This mismatch means a completely filled buffer
cannot be read.  Fix this by making the size checks identical.

Signed-off-by: Jason Andryuk <andryuk@aero.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>