restapi: Add snapshot id to snapshot preview RSDL

The snapshot preview action requires a snapshot id for each disk, but
this isn't currently documented in the RSDL metadata. This patch adds
the documentation.

Change-Id: I11b0b4731d127496b1c9622c54fd3fe48309d9f3
Bug-Url: https://bugzilla.redhat.com/1150071
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>