x86/paravirt: Introduce ALT_NOT_XEN

Introduce the macro ALT_NOT_XEN as a short form of
ALT_NOT(X86_FEATURE_XENPV).

No functional changes.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231129133332.31043-2-jgross@suse.com