x86/HVM: don't retain emulated insn cache when exiting back to guest

x86/HVM: don't retain emulated insn cache when exiting back to guest

vio->mmio_retry is being set when a repeated string insn is being split
up. In that case we'll exit to the guest, expecting immediate re-entry.
Interruptions, however, may be serviced by the guest before re-entry
from the repeated string insn. Any emulation needed in the course of
handling the interruption must not fetch from the internally maintained
cache.

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