run: qemu - Add support to enable vhost

Make it more convenient to enable vhost=on or vhostforce=on
using the test runner. As of now, bridge access still
requires root, so you'd need to run things as root:

sudo ./run -t qemu --nettype bridge --vhost on

or

sudo ./run -t qemu --nettype bridge --vhost force

CC: Richa Marwha <rmarwah@us.ibm.com>
CC: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>