x86/hvm: fix interaction between internal and external emulation

x86/hvm: fix interaction between internal and external emulation

A call to handle_hvm_io_completion() is needed for completing I/O
that requires external emulation. Such completion should be requested when
hvm_vcpu_io_need_completion() returns true after hvm_emulate_once() has
completed. This is indicative of the underlying I/O emulation having
returned X86EMUL_RETRY and hence a re-emulation of the instruction is
needed to pick up the result of the I/O.

A call to handle_hvm_io_completion() is NOT needed when the underlying
I/O has not returned X86EMUL_RETRY since there will be no result to pick
up. Hence it bogus to request such completion when mmio_retry is set,
since this can only happen if the underlying I/O emulation has returned
X86EMUL_OKAY (meaning the I/O has completed successfully).

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>