argo: correctly report pending message length

argo: correctly report pending message length

When a message is requeue'd in Xen's internal queue, the queue
entry contains the length of the message so that Xen knows to
send a VIRQ to the respective domain when enough space frees up
in the ring. Due to a small bug, however, Xen doesn't populate
the length of the msg if a given write fails, so this length is
always reported as zero. This causes Xen to spuriously wake up
a domain even when the ring doesn't have enough space.

This patch makes sure that the msg len is properly reported by
populating it in the event of a write failure.

Signed-off-by: Nicholas Tsirakis <tsirakisn@ainfosec.com>
Reviewed-by: Christopher Clark <christopher.w.clark@gmail.com>