x86/xen: fill hypercall page with int3

Filling the hypercall page with nops is not helpful from a debugging point of
view, as for example attempting to execute an hypercall before the page is
initialized will result in the execution flow falling through into
xen_start32, making the mistake less obvious to spot.

Instead fill the page with int3 (0xcc) which will result in a #BP trap.

Sponsored by: Cloud Software Group
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D43930