x86/boot: Don't enable EFER.SCE for !CONFIG_PV builds

x86/boot: Don't enable EFER.SCE for !CONFIG_PV builds

This will cause all SYSCALL/SYSRET instructions to suffer #UD rather than
following the MSR_{L,C}STAR pointers, allowing us to drop the star_enter()
panic helper, allowing us to clean up the IST stacks in a subsequent patch.

Drop the now-dead conditional SYSENTER logic in the middle of
subarch_percpu_traps_init().

In addition, vmx_restore_host_msrs() need not restore any host
state.  (Regarding the asymmetric changes, VT-x automatically restores
SYSENTER state on vmexit, and SVM restores both SYSCALL/SYSENTER state with
the VMSAVE/VMLOAD instructions.)

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