DOCUMENTATION: Protected virtual machine introduction and IPL

Add documentation about protected KVM guests and description of changes
that are necessary to move a KVM VM into Protected Virtualization mode.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
[borntraeger@de.ibm.com: fixing and conversion to rst]
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>