vhost: add debug logging

Log when mbufs cannot be enqueued to vhost port.

Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>

Change-Id: Iaac535cd7cecca7c2e2a1c79dc318b3a4b56c82b
Reviewed-By: Mark Gray <mark.d.gray@intel.com>