When checking for a valid network, we weren't consistent on
whether we reported an invalid network or a connection.  Similar
to previous patches such as commit 6e130ddc, the difference
between VIR_IS_NETWORK and VIR_IS_CONNECTED_NETWORK is moot (due
to reference counting, any valid network must be tied to a valid
connection).  Use a common macro to make the error reporting
for invalid networks nicer.

* src/datatypes.h (virCheckNetworkReturn, virCheckNetworkGoto): New
macros.
(VIR_IS_NETWORK, VIR_IS_CONNECTED_NETWORK): Drop unused macros.
* src/libvirt.c: Use macro throughout.
(virLibNetworkError): Drop unused macro.
Signed-off-by: NEric Blake <eblake@redhat.com>