x86/HVM: use available linear->phys translations in REP MOVS/STOS handling

x86/HVM: use available linear->phys translations in REP MOVS/STOS handling

If we have the translation result available already, we should also use
it here. In my tests with Linux guests this eliminates all calls to
hvmemul_linear_to_phys() from the STOS path and most from the MOVS one.

Also record the translation for re-use at least during response
processing.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>