restapi: Add VM pool description to RSDL

The "description" property of the operation to update a VM pool isn't
documented in the metadata file, thus it doesn't appear in the RSDL
document and isn't available in the CLI.

Change-Id: If0ece6b092eed72ccb1fa04ea05308f046e5755e
Bug-Url: https://bugzilla.redhat.com/1085386
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>