While all errors related to invalid nwfilters appeared to be
consistent, we might as well continue the trend of using a
common macro.  As in commit 6e130ddc, the difference between
VIR_IS_NWFILTER and VIR_IS_CONNECTED_NWFILTER is moot, since
reference counting means any valid nwfilter is also tied to
a valid connection.  For now, we don't need virCheckNWFilterGoto().

* src/datatypes.h (virCheckNWFilterReturn): New macro.
(VIR_IS_NWFILTER, VIR_IS_CONNECTED_NWFILTER): Drop unused macros.
* src/libvirt.c: Use macro throughout.
(virLibNWFilterError): Drop unused macro.
Signed-off-by: NEric Blake <eblake@redhat.com>