CA-129192: restore metadata for all SR

CA-129192/SCTX-1728: revise restore-sr-metadata.py so that it could restore
metadata for all SR with command line:
   ./restore-sr-metadata.py -f <input file> -u all
or just omit the sr-uuid:
   ./restore-sr-metadata.py -f <input file>

xe-restore-metadata is modified accordingly.

Signed-off-by: Kaifeng Zhu <kaifeng.zhu@citrix.com>
Merge pull request #1685 from johnelse/ca-129192

CA-129192: restore metadata for all SR