hvm/fep: Allow testing of instructions crossing the -1 -> 0 virtual boundary

hvm/fep: Allow testing of instructions crossing the -1 -> 0 virtual boundary

The Force Emulation Prefix is named to follow its PV counterpart for cpuid or
rdtsc, but isn't really an instruction prefix.  It behaves as a break-out into
Xen, with the purpose of emulating the next instruction in the current state.

It is important to be able to test legal situations which occur in real
hardware, including instruction which cross certain boundaries, and
instructions starting at 0.

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