CA-129192: Restore metadata for all SR

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>