mac80211_hwsim: allow creating/destroying radios on the fly

Add new commands to the hwsim generic netlink family to allow
creating and destroying radios on the fly. The number of channels
a radio supports can be specified when it's created, if it isn't
the module parameter will be used as default.

This should be extended in the future to allow other parameters
to be specified, e.g.
 * list of channels
 * interface combinations, particularly P2P_DEVICE support
 * regtest
 * and pretty much all other hardware capabilities

Signed-off-by: Johannes Berg <johannes.berg@intel.com>