x86: introduce a new VMASSIST for architectural behaviour of iopl

x86: introduce a new VMASSIST for architectural behaviour of iopl

The existing vIOPL interface is hard to use, and need not be.

Introduce a VMASSIST with which a guest can opt-in to having vIOPL behaviour
consistenly with native hardware.

Specifically:
 - virtual iopl updated from do_iret() hypercalls.
 - virtual iopl reported in bounce frames.
 - guest kernels assumed to be level 0 for the purpose of iopl checks.

v->arch.pv_vcpu.iopl is altered to store IOPL shifted as it would exist
eflags, for the benefit of the assembly code.

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