jsonrpc: StoragePool_getBackedUpVmsList

Align naming between API.py and schema.

Change-Id: Iad5db03580000b4ec219432a1441294ac1a03286
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35546
Reviewed-by: Adam Litke <alitke@redhat.com>