block/nfs: fix QMP to match debug option

block/nfs: fix QMP to match debug option

The QMP definition of BlockdevOptionsNfs:
{ 'struct': 'BlockdevOptionsNfs',
  'data': { 'server': 'NFSServer',
            'path': 'str',
            '*user': 'int',
            '*group': 'int',
            '*tcp-syn-count': 'int',
            '*readahead-size': 'int',
            '*page-cache-size': 'int',
            '*debug-level': 'int' } }

To make this consistent with other block protocols like gluster, lets
change s/debug-level/debug/

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>