x86/smt: Support for enabling/disabling SMT at runtime

x86/smt: Support for enabling/disabling SMT at runtime

Currently, a user can in principle combine the output of `xl info -n`, the
APCI tables, and some manual CPUID data to figure out which CPU numbers to
feed into `xen-hptool cpu-offline` to effectively disable SMT at runtime.

A more convenient option is to teach Xen how to perform this action.

Extend XEN_SYSCTL_cpu_hotplug with two new operations.  Introduce a new
smt_up_down_helper() which wraps the cpu_{up,down}_helper() helpers with logic
which understands siblings based on their APIC_ID.

Add libxc stubs, and extend xen-hptool with smt-{enable,disable} options.
These are intended to be shorthands for a loop over cpu-{online,offline}.

To simplify the implemention, they will strictly enable/disable secondary
siblings (those with a non-zero thread id).  This functionality is intended
for use in production scenarios where debugging options such as `maxcpus=` or
other manual plug/unplug configuration has not been used.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>