x86/xen: Use specific Xen pv interrupt entry for MCE

Xen PV guests don't use IST. For machine check interrupts, switch to the
same model as debug interrupts.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210120135555.32594-3-jgross@suse.com