x86/PV: use generic emulator for privileged instruction handling

x86/PV: use generic emulator for privileged instruction handling

There's a new emulator return code being added to allow bypassing
certain operations (see the code comment).

Another small tweak to the emulator is to single iteration handling
of INS and OUTS: Since we don't want to handle any other memory access
instructions, we want these to be handled by the rep_ins() / rep_outs()
hooks here too.

And then long-mode related bits now get hidden from the guest. This
should have been that way from the beginning, but becomes a requirement
now as the emulator's in_longmode() needs this to reflect guest view.

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