[ACM] Support for removing / replacing a policy

[ACM] Support for removing / replacing a policy

Add support for removing/replacing a policy from the running
system. This operation is only successful if currently running VMs
would also be allowed to run under the new policy. Removing the
current policy means that the default policy is installed, which then
only has support for a single VM label and STE type (SystemManagement)
and is the same policy that the system starts up with when no policy
is chosen.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>