x86: introduce CONFIG_GUEST and move code

x86: introduce CONFIG_GUEST and move code

Xen is able to run as a guest on Xen. We plan to make it able to run
on Hyper-V as well.

Introduce CONFIG_GUEST which is set to true if either running on Xen
or Hyper-V is desired. Restructure code hierarchy for new code to
come.

No functional change intended.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>