env_process: Free mac address when destroying VM

Sometimes we meet error "NetError: qemu/default MAC generation
failed with prefix 9a after 1024 attempts for NIC nic1 on VM virt-tests-vm1

We should free the mac address when destroying VM. Then we can
reuse the mac in other case.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1469 from FengYang/free_mac

env_process: Free mac address when destroying VM