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>
qemu.tests.cfg: Enable usb_nodev variant on RHEL host

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
shared.cfg: Fix the s4_support_chk_cmd for Fedora guest

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Merge pull request #616 from zhouqt/minor-fix

Qemu: Minor fix for some config of qemu test cases
Merge pull request #579 from FengYang/vhostfd

virttest.qemu_vm: support vhostfd in netdev.
utils_libvirtd: if libvirt is not availible in host, raise ImportError.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>