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

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>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>