Check whether pools are already active upon libvirtd startup

Check whether pools are already active upon libvirtd startup

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