Description of problem:
 When we try to use vhost-user, but the QEMU binary has the feature compiled out - like qemu-kvm and qemu-kvm-ma, libvirt spawns QEMU message directly. This can be enhanced in libvirt to give more meaningful message.

Version-Release number of selected component (if applicable):
qemu-kvm-ma-2.10.0-17.el7.ppc64le
libvirt-3.9.0-9.el7.ppc64le
kernel-4.14.0-25.el7a.ppc64le

How reproducible:
100%

Steps to Reproduce:
1. Edit /etc/default/grub
Append "numa=off" and "hugepages=1024" (enable the hugepage and disable numa)
2. Run "grub2-mkconfig -o /boot/grub2/grub.cfg"
3. Reboot the guest( Run "reboot")
4. Set selinux
Run "setenforce 0 " (bug1278495)
5. Run the shell scripts:
***********************
#!/bin/bash

set -e
set -x
echo "killing old ovs process"
pkill -f ovs- || true
pkill -f ovsdb || true

echo "probing ovs kernel module"
modprobe -r openvswitch || true
modprobe openvswitch

echo "clean env"
DB_FILE=/etc/openvswitch/conf.db
rm -rf /var/run/openvswitch
mkdir /var/run/openvswitch
rm -f $DB_FILE

echo "init ovs db and boot db server"
export DB_SOCK=/var/run/openvswitch/db.sock
ovsdb-tool create /etc/openvswitch/conf.db /usr/share/openvswitch/vswitch.ovsschema
ovsdb-server --remote=punix:$DB_SOCK --remote=db:Open_vSwitch,Open_vSwitch,manager_options --pidfile --detach --log-file
ovs-vsctl --no-wait init

echo "start ovs vswitch daemon"
ovs-vsctl --no-wait set Open_vSwitch . other_config:dpdk-init=true
ovs-vsctl --no-wait set Open_vSwitch . other_config:dpdk-socket-mem="4096"
ovs-vsctl --no-wait set Open_vSwitch . other_config:dpdk-lcore-mask="0x1010000000000"
ovs-vswitchd unix:$DB_SOCK --pidfile --detach --log-file=/var/log/openvswitch/ovs-vswitchd.log --verbose

echo "creating bridge and ports"
ovs-vsctl --if-exists del-br br0
ovs-vsctl add-br br0 -- set bridge br0 datapath_type=netdev 
ovs-vsctl add-port br0 vhost-vm1 -- set Interface vhost-vm1 type=dpdkvhostuser 
ovs-vsctl add-port br0 vhost-vm2 -- set Interface vhost-vm2 type=dpdkvhostuser
ip add add 192.168.2.1/24 dev br0; ip link set dev br0 up

***********************
# chown qemu:qemu /var/run/openvswitch/vhost-vm*

6. Define one guest (Reminder: the guest name. the disk source, the interface mac, the interface source path need to change) (Warning: "mrg_rxbuf='off'" is a must)
***************************
<domain type='kvm'>
  <name>vm1</name>
  <memory unit='KiB'>3145728</memory>
  <currentMemory unit='KiB'>3145728</currentMemory>
  <memoryBacking>
    <hugepages>
      <page size='16' unit='M' nodeset='0'/>
    </hugepages>
  </memoryBacking>
  <vcpu placement='static'>2</vcpu>
  <os>
    <type arch='ppc64le' machine='pseries'>hvm</type>
    <boot dev='hd'/>
    <boot dev='cdrom'/>
  </os>
  <cpu mode='host-model'>
    <model fallback='allow'/>
    <topology sockets='2' cores='1' threads='1'/>
    <numa>
      <cell id='0' cpus='0-1' memory='3145728' unit='KiB' memAccess='shared'/>
    </numa>
  </cpu>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='?????????????????.qcow2'/>
      <target dev='vda' bus='virtio'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <target dev='sda' bus='scsi'/>
      <readonly/>
    </disk>
    <interface type='vhostuser'>
      <mac address='00:00:00:00:00:01'/>
      <source type='unix' path='/var/run/openvswitch/vhost-vm1' mode='client'/>
       <model type='virtio'/>
      <driver queues='2'>
        <host mrg_rxbuf='off'/>
      </driver>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
  </devices>
</domain>
*******************

7. Start the guest
# virsh start dpdk1
error: Failed to start domain dpdk1
error: internal error: qemu unexpectedly closed the monitor: 2018-01-25T08:40:41.859965Z qemu-kvm: -netdev vhost-user,chardev=charnet0,queues=2,id=hostnet0: Parameter 'type' expects a netdev backend type

Actual results:
See step 7.

Expected results:
Libvirt can display more meaningful message.

Additional info: