KVM: implement the HT_SM_POOL security model

In order not to complicate to much the _ExecuteKVMRuntime function with
nested try/except/finally/else constructs we move the actual
runcmd+check call in a separate _RunKVMCmd function.

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