Introduce 'global hypervisor parameters' support

This patch adds support for global hypervisor parameters in instance
creation, instance modification, instance query and at instance load
time.

We basically prevent any query on these parameters, discard them at load
time, and do not allow their modification. Together, this should make
any such parameters go away if existing and not allowed to be added.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>