virt: Fix ./run vhost params.

run uses for setting vhost to network netdev_extra_params.
This param has to be used with ',' in-front of network param when
it is used for standard thinks. In future it should be replaced by
new vhost virt_net interface.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
Merge pull request #776 from jzupka/runner

virt: Fix ./run vhost params.