x86/domctl: Implement XEN_DOMCTL_get_cpu_policy

x86/domctl: Implement XEN_DOMCTL_get_cpu_policy

This finally (after literally years of work!) marks the point where the
toolstack can ask the hypervisor for the current CPUID configuration of a
specific domain.

Introduce a new flask access vector and update the default policies.

Also extend xen-cpuid's --policy mode to be able to take a domid and dump a
specific domains CPUID and MSR policy.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>