x86/xen: move paravirt lazy code

x86/xen: move paravirt lazy code

Only Xen is using the paravirt lazy mode code, so it can be moved to
Xen specific sources.

This allows to make some of the functions static or to merge them into
their only call sites.

While at it do a rename from "paravirt" to "xen" for all moved
specifiers.

No functional change.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20230913113828.18421-3-jgross@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>