virttest.env_process: update vm.virtnet when vm is not alive

If vm is not alive return true when check  vm needs_restart to avoid incoming case use an old vm that create to negative test.
Used default params to overwrite default  value for nic properties which reading from DB to avoid invalid nic param impact income testing which not set that properties in config.   

Signed-off-by: Xu Tian <xutian@redhat.com>