SVM: streamline entry.S code

SVM: streamline entry.S code

- fix a bogus "test" with zero immediate
- move stuff easily/better done in C into C code
- re-arrange code paths so that no redundant GET_CURRENT() would remain
  on the fast paths
- move long latency operations earlier
- slightly defer disabling global interrupts on the VM entry path

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