xen/xsplice: Hypervisor implementation of XEN_XSPLICE_op

xen/xsplice: Hypervisor implementation of XEN_XSPLICE_op

The implementation does not actually do any patching.

It just adds the framework for doing the hypercalls,
keeping track of ELF payloads, and the basic operations:
 - query which payloads exist,
 - query for specific payloads,
 - check*1, apply*1, replace*1, and unload payloads.

*1: Which of course in this patch are nops.

The functionality is disabled on ARM until all arch
components are implemented.

Also by default it is disabled until the implementation
is in place.

We also use recursive spinlocks to so that the find_payload
function does not need to have a 'lock' and 'non-lock' variant.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>