storage: Initialize pool size parameters for refresh thread

If the pool creation thread happens to detect the luns in
the scsi target, the size parameters will be calculated as
part of the refreshPool called from storagePoolCreate().

This means the virStoragePoolFCRefreshThread (commit id
'512b874') waiting to run and "refresh" the pool will
essentially double the allocation and capacity values.
A separate refresh would correct the values.

To avoid this, the FCRefreshThread needs to reinitialize
the pool size values prior to calling virStorageBackendSCSIFindLUs
which eventually calls virStorageBackendSCSINewLun and
updates the size values for each volume found.

src/storage/storage_backend_scsi.c

@@ -614,6 +614,9 @@ virStoragePoolFCRefreshThread(void *opaque)
          */
         VIR_DEBUG("Attempt FC Refresh for pool='%s' name='%s' tries='%d'",
                   pool->def->name, fchost_name, tries);
+
+        pool->def->allocation = pool->def->capacity = pool->def->available = 0;
+
         if (virStoragePoolObjIsActive(pool) &&
             virGetSCSIHostNumber(fchost_name, &host) == 0 &&
             virStorageBackendSCSITriggerRescan(host) == 0) {