x86-64: drop updating of UREGS_rip when converting sysenter to #GP

x86-64: drop updating of UREGS_rip when converting sysenter to #GP

This was set to zero immediately before the #GP injection code, since
SYSENTER doesn't really have a return address.

Reported-by: Ian Campbell <Ian.Campbell@citrix.com>
Furthermore, UREGS_cs and UREGS_rip don't need to be written a second
time, as the PUSHes above already can/do take care of putting in place
the intended values.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>