The meat of this patch is just moving the calls to
virNWFilterRegisterCallbackDriver from each hypervisor's "register"
function into its "initialize" function. The rest is just code
movement to allow that, and a new virNWFilterUnRegisterCallbackDriver
function to undo what the register function does.

The long explanation:

There is an array in nwfilter called callbackDrvArray that has
pointers to a table of functions for each hypervisor driver that are
called by nwfilter. One of those function pointers is to a function
that will lock the hypervisor driver. Entries are added to the table
by calling each driver's "register" function, which happens quite
early in libvirtd's startup.

Sometime later, each driver's "initialize" function is called. This
function allocates a driver object and stores a pointer to it in a
static variable that was previously initialized to NULL. (and here's
the important part...) If the "initialize" function fails, the driver
object is freed, and that pointer set back to NULL (but the entry in
nwfilter's callbackDrvArray is still there).

When the "lock the driver" function mentioned above is called, it
assumes that the driver was successfully loaded, so it blindly tries
to call virMutexLock on "driver->lock".

BUT, if the initialize never happened, or if it failed, "driver" is
NULL. And it just happens that "lock" is always the first field in
driver so it is also NULL.

Boom.

To fix this, the call to virNWFilterRegisterCallbackDriver for each
driver shouldn't be called until the end of its (*already guaranteed
successful*) "initialize" function, not during its "register" function
(which is currently the case). This implies that there should also be
a virNWFilterUnregisterCallbackDriver() function that is called in a
driver's "shutdown" function (although in practice, that function is
currently never called).