x86/hvmloader: Don't wait for the producer to fill the ring if

x86/hvmloader: Don't wait for the producer to fill the ring if

The condition: if there is a space in the ring then wait for the producer
to fill the ring also evaluates to true even if the ring if full. It
leads to a deadlock where producer is waiting for consumer
to consume the items and consumer is waiting for producer to fill the ring.

Fix for the issue: check if the ring is full and then break from
the loop to consume the items from the ring.
eg. case: prod = 1272, cons = 248.

Signed-off-by: Anshul Makkar <anshul.makkar@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>