x86/emul: Poision the stubs with debug traps

x86/emul: Poision the stubs with debug traps

...rather than leaving fragments of old instructions in place.  This reduces
the chances of something going further-wrong (as the debug trap will be caught
and terminate the guest) in a cascade-failure where we end up executing the
instruction fragments.

Before:
    (XEN) d2v0 exception 6 (ec=0000) in emulation stub (line 6239)
    (XEN) d2v0 stub: c4 e1 44 77 c3 80 d0 82 ff ff ff d1 90 ec 90

After:
    (XEN) d3v0 exception 6 (ec=0000) in emulation stub (line 6239)
    (XEN) d3v0 stub: c4 e1 44 77 c3 cc cc cc cc cc cc cc cc cc cc

To make this work, the int3 handler needs to be extended to attempt recovery
rather than simply returning back to Xen context.  While altering do_int3(),
leave an obvious sign if an embedded breakpoint has been hit and not dealt
with by debugging facilities.

    (XEN) Hit embedded breakpoint at ffff82d0803d01f6 [extable.c#stub_selftest+0xda/0xee]

Extend the selftests to include int3, and add an extra printk indicating the
start of the recovery selftests, to avoid leaving otherwise-spurious faults
visible in the log.

    (XEN) build-id: 55d7e6f420b4f0ce277f776be620f43d7cb8646c
    (XEN) Running stub recovery selftests...
    (XEN) traps.c:3466: GPF (0000): ffff82d0bffff041 [ffff82d0bffff041] -> ffff82d08035937a
    (XEN) traps.c:813: Trap 12: ffff82d0bffff040 [ffff82d0bffff040] -> ffff82d08035937a
    (XEN) traps.c:1215: Trap 3: ffff82d0bffff041 [ffff82d0bffff041] -> ffff82d08035937a
    (XEN) ACPI sleep modes: S3

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>