mac80211_hwsim: add frame transmission support over virtio

This allows communication with external entities.

It also required fixing up the netlink policy, since NLA_UNSPEC
attributes are no longer accepted.

Signed-off-by: Erel Geron <erelx.geron@intel.com>
[port to backports, inline the ID, use 29 as the ID as requested,
 drop != NULL checks, reduce ifdefs]
Link: https://lore.kernel.org/r/20200305143212.c6e4c87d225b.I7ce60bf143e863dcdf0fb8040aab7168ba549b99@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>