Currently, qemu driver uses qemuTranslateDiskSourcePool()
to translate disk volume information. This function is
general enough and could be used for other drivers as well,
so move it to conf/domain_conf.c along with its helpers.

 - qemuTranslateDiskSourcePool: move to storage/storage_driver.c
   and rename to virStorageTranslateDiskSourcePool,
 - qemuAddISCSIPoolSourceHost: move to storage/storage_driver.c
   and rename to virStorageAddISCSIPoolSourceHost,
 - qemuTranslateDiskSourcePoolAuth: move to storage/storage_driver.c
   and rename to virStorageTranslateDiskSourcePoolAuth,
 - Update users of qemuTranslateDiskSourcePool to use a
   new name.