When libvirt starts up all storage pools default to the inactive
state, even if the underlying storage is already active on the
host. This introduces a new API into the internal storage backend
drivers that checks whether a storage pool is already active. If
the pool is active at libvirtd startup, the volume list will be
immediately populated.

* src/storage/storage_backend.h: New internal API for checking
  storage pool state
* src/storage/storage_driver.c: Check whether a pool is active
  upon driver startup
* src/storage/storage_backend_fs.c, src/storage/storage_backend_iscsi.c,
  src/storage/storage_backend_logical.c, src/storage/storage_backend_mpath.c,
  src/storage/storage_backend_scsi.c: Add checks for pool state