restapi : Error when adding/removing permissions with filtered api(#919686)

Internal server error when UserVmManager adds a User role permission on a VM,
the permission gets added.

Insufficent privileges error when removing the permission which was added.

Change-Id: I47cbb6e06599fc64afcb491bef15b46a82afd681
Bug-Url: https://bugzilla.redhat.com/919686
Signed-off-by: Ravi Nori <rnori@redhat.com>