vhost: do not stall if guest is slow

vhost: do not stall if guest is slow

When guest is booting (or any othertime guest is busy) it is possible
for the small receive ring (256) to get full. If this happens the
vhost library should just return normally. It's current behavior
of logging just creates massive log spew/overflow which could even
act as a DoS attack against host.

Reported-by: Nathan Law <nlaw@brocade.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>