jsonrpc: StoragePool_getBackedUpVmsInfo

Align naming between API.py and schema.

Change-Id: Id66b8b28602c76d44a525299bcaf6aec1697073d
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35545
Reviewed-by: Adam Litke <alitke@redhat.com>