x86/cpu: Sysctl and common infrastructure for levelling context switching

x86/cpu: Sysctl and common infrastructure for levelling context switching

A toolstack needs to know how much control Xen has over the visible cpuid
values in PV guests.  Provide an explicit mechanism to query what Xen is
capable of.

This interface will currently report no capabilities.  This change is
scaffolding for future patches, which will introduce detection and switching
logic, after which the interface will report hardware capabilities correctly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <JBeulich@suse.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>