maint: improve VIR_ERR_INVALID_INTERFACE usage

maint: improve VIR_ERR_INVALID_INTERFACE usage

When checking for a valid interface, we weren't consistent on
whether we reported as VIR_FROM_NONE or VIR_FROM_INTERFACE.
Similar to previous patches, use a common macro to make it nicer.
Furthermore, just as in commit 6e130ddc, the difference between
VIR_IS_INTERFACE and VIR_IS_CONNECTED_INTERFACE is moot (due to
reference counting, any valid interface must be tied to a valid
connection).  For now, we don't need virCheckInterfaceGoto().

* src/datatypes.h (virCheckInterfaceReturn): New macro.
(VIR_IS_INTERFACE, VIR_IS_CONNECTED_INTERFACE): Drop unused
macros.
* src/libvirt.c: Use macro throughout.
(virLibInterfaceError): Drop unused macro.

Signed-off-by: Eric Blake <eblake@redhat.com>