vhost-user: fix missing speculative enqueue unwind
vhost-user: fix missing speculative enqueue unwind

Running trex in a VM with a bad config, trex sent a bogus pack from
the VM to the Virtual interface. It caused a crash.

Change-Id: I64d0197b444265553ab4c24f21e6a962e89cb587
Signed-off-by: Steven <a748ea44ad7b179c2313a856d44e1e4eb39869bb@cisco.com>

FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,milanlenco/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,milanlenco/vpp,milanlenco/vpp,milanlenco/vpp,vpp-dev/vpp,FDio/vpp,milanlenco/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,milanlenco/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp