qdict: Introduce qdict_rename_keys()

A few block drivers will need to rename .bdrv_create options for their
QAPIfication, so let's have a helper function for that.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>