xen: add runtime parameter access support to hypfs

xen: add runtime parameter access support to hypfs

Add support to read and modify values of hypervisor runtime parameters
via the hypervisor file system.

As runtime parameters can be modified via a sysctl, too, this path has
to take the hypfs rw_lock as writer.

For custom runtime parameters the connection between the parameter
value and the file system is done via an init function which will set
the initial value (if needed) and the leaf properties.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>