virStoragePoolBuild reported an invalid pool as if it were an
invalid network.  Likewise, we weren't consistent on whether to
use VIR_FROM_NONE or VIR_FROM_STORAGE.  Similar to previous
patches, use a common macro to make it nicer.  Furthermore, just
as in commit 6e130ddc, the difference between VIR_IS_STORAGE_POOL
and VIR_IS_CONNECTED_STORAGE_POOL is moot (due to reference
counting, any valid pool must be tied to a valid connection).
For now, we don't need virCheckStoragePoolGoto().

* src/datatypes.h (virCheckStoragePoolReturn): New macro.
(VIR_IS_STORAGE_POOL, VIR_IS_CONNECTED_STORAGE_POOL): Drop
unused macros.
* src/libvirt.c: Use macro throughout.
(virLibStoragePoolError): Drop unused macro.
Signed-off-by: NEric Blake <eblake@redhat.com>