qemu.qemu_vm: support vhostfd in netdev.

vhostfd only works when vhost on.
add enable_vhost parameter. set default value to 'yes'
Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #579 from FengYang/vhostfd

virttest.qemu_vm: support vhostfd in netdev.