virttest.virt_vm: check vm alive in need_restart function

If vm is not alive return True, and a new VM object in prepare env process
to avoid new case use a inviald VM.

Signed-off-by: Xu Tian <xutian@redhat.com>
virttest.utils_net: set defaults for nic params object

We can use all  properties of nic in previous as default for incoimg VM,
if previous nic use specail configure which will impact all next case
which not set default params in cfg.

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #1894 from xutian/fix_add_port2br

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