x86/hvm: remove HVMIO_dispatched I/O state

x86/hvm: remove HVMIO_dispatched I/O state

By removing the HVMIO_dispatched state and making all pending emulations
(i.e. all those not handled by the hypervisor) use HVMIO_awating_completion,
various code-paths can be simplified.

The completion case for HVMIO_dispatched can also be trivally removed
from hvmemul_do_io() as it was already unreachable. This is because that
state was only ever used for writes or I/O to/from a guest page and
hvmemul_do_io() is never called to complete such I/O.

NOTE: There is one sublety in handle_pio()... The only case when
      handle_pio() got a return code of X86EMUL_RETRY back from
      hvmemul_do_pio_buffer() and found io_state was not
      HVMIO_awaiting_completion was in the case where the domain is
      shutting down. This is because all writes normally yield a return
      of HVMEMUL_OKAY and all reads put io_state into
      HVMIO_awaiting_completion. Hence the io_state check there is
      replaced with a check of the is_shutting_down flag on the domain.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>