storage: make disk source pool translation generic

storage: make disk source pool translation generic

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.