x86/pv: Drop int80_bounce from struct pv_vcpu

x86/pv: Drop int80_bounce from struct pv_vcpu

The int80_bounce field of struct pv_vcpu is a bit of an odd special case,
because it is a simple derivation of trap_ctxt[0x80], which is also stored.

It is also the only use of {compat_,}create_bounce_frame() which isn't
referencing the plain trap_bounce field of struct pv_vcpu.  (And altering this
property the purpose of this patch.)

Remove the int80_bounce field entirely, along with init_int80_direct_trap(),
which in turn requires that the int80_direct_trap() path gain logic previously
contained in init_int80_direct_trap().

This does admittedly make the int80 fastpath slightly longer, but these few
instructions are in the noise compared to the architectural context switch
overhead, and it now matches the syscall/sysenter paths (which have far less
architectural overhead already).

No behavioural change from the guests point of view.

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