Disable nwfilter driver when running unprivileged

Disable nwfilter driver when running unprivileged

When opening a new connection to the driver, nwfilterOpen
only succeeds if the driverState has been allocated.

Move the privilege check in driver initialization before
the state allocation to disable the driver.

This changes the nwfilter-define error from:
error: cannot create config directory (null): Bad address
To:
this function is not supported by the connection driver:
virNWFilterDefineXML

https://bugzilla.redhat.com/show_bug.cgi?id=1029266
(cherry picked from commit b7829f959b33c6e32422222a9ed745c0da7dc696)