vhost: fail I/Os with payload length = 0

We assumed the second descriptor in an I/O descriptor
chain will always point to a payload buffer, but in case
there is no payload, the second descriptor will point to
a response buffer. The vhost code doesn't provide proper
checks to handle such case, so to avoid various errors
down the stack, we just fail all requests with no
payload.

Change-Id: I6785c2843d6db4fc17e68e03562c2a1530bb469b
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/437187


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by:  <dstepanov.src@gmail.com>