x86/hvm: don't leave emulator in inconsistent state

The fact that handle_mmio(), and thus the instruction emulator, is
being run through twice for emulations that require involvement of the
device model, allows for the second run to see a different guest state
than the first one. Since only the MMIO-specific emulation routines
update the vCPU's io_state, if they get invoked on the second pass,
internal state (and particularly this variable) can be left in a state
making successful emulation of a subsequent MMIO operation impossible.

Consequently, whenever the emulator invocation returns without
requesting a retry of the guest instruction, reset io_state.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>

--- a/xen/arch/x86/hvm/io.c
+++ b/xen/arch/x86/hvm/io.c
@@ -179,6 +179,8 @@ int handle_mmio(void)
 
     rc = hvm_emulate_one(&ctxt);
 
+    if ( rc != X86EMUL_RETRY )
+        vio->io_state = HVMIO_none;
     if ( vio->io_state == HVMIO_awaiting_completion )
         vio->io_state = HVMIO_handle_mmio_awaiting_completion;
     else