restapi: Remove /api from snapshot metadata URL

Currently the operations related to snapshots at the VM level contain an
extra /api prefix in the specification of the URL. This confuses the
generator of the RSDL metadata and needs to be removed.

Change-Id: I672aee761ad9b310cc129ce4eabebe3745b85f59
Bug-Url: https://bugzilla.redhat.com/1101405
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>