x86/pv: Fix bugs with the handling of int80_bounce

x86/pv: Fix bugs with the handling of int80_bounce

Testing has revealed two issues:

 1) Passing a NULL handle to set_trap_table() is intended to flush the entire
    table.  The 64bit guest case (and 32bit guest on 32bit Xen, when it
    existed) called init_int80_direct_trap() to reset int80_bounce, but c/s
    cda335c279 which introduced the 32bit guest on 64bit Xen support omitted
    this step.  Previously therefore, it was impossible for a 32bit guest to
    reset its registered int80_bounce details.

 2) init_int80_direct_trap() doesn't honour the guests request to have
    interrupts disabled on entry.  PVops Linux requests that interrupts are
    disabled, but Xen currently leaves them enabled when following the int80
    fastpath.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>