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

Xen PV guests don't use IST. For double fault interrupts, switch to
the same model as NMI.

Correct a typo in a comment while copying it.

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/20210120135555.32594-4-jgross@suse.com