restapi: Fix update VM optional parameters

The metadata for the operation to update a VM doesn't include the
guaranteed memory option, thus it isn't available in the CLI.

Change-Id: I5d32fb2db5bf4b9f2f3271820ce7b0caeac9e050
Bug-Url: https://bugzilla.redhat.com/1082594
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>