docs: Describe protected virtualization guest setup

docs: Describe protected virtualization guest setup

Protected virtualization/IBM Secure Execution for Linux protects
guest memory and state from the host.

Add some basic information about technology and a brief guide
on setting up secure guests with libvirt.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Signed-off-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Paulo de Rezende Pinatti <ppinatti@linux.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>