SVM: avoid VMSAVE in ctxt-switch-to

SVM: avoid VMSAVE in ctxt-switch-to

Of the state saved by the insn and reloaded by the corresponding VMLOAD
- TR and syscall state are invariant while having Xen's state loaded,
- sysenter is unused altogether by Xen,
- FS, GS, and LDTR are not used by Xen and get suitably set in PV
  context switch code.
Note that state is suitably populated in _svm_cpu_up(); a minimal
respective assertion gets added.

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