Some fixes to IRET hypercall and failsafe callback handlers:

Some fixes to IRET hypercall and failsafe callback handlers:
 1. IRET hypercall must restore the event callback mask. This
    was missing on x86/64, and both subarchitectures now restore
    from EFLAGS.IF.
 2. Failsafe callbacks are fixed to detect whether fault is due to
    bad segment or due to IRET. In the latter case we now
    immediatiately kill the process. This avoids infinite looping
    between IRET hypercall and failsafe callback handler in the guest.

Signed-off-by: Keir Fraser <keir@xensource.com>