x86/hvm: only call hvm_io_assist() from hvm_wait_for_io()

x86/hvm: only call hvm_io_assist() from hvm_wait_for_io()

By removing the calls in hvmemul_do_io() (which is replaced by a single
assignment) and hvm_complete_assist_request() (which is replaced by a
call to hvm_process_portio_intercept() with a suitable set of ops) then
hvm_io_assist() can be moved into hvm.c and made static (and hence be a
candidate for inlining).

The calls to msix_write_completion() and vcpu_end_shutdown_deferral()
are also made unconditionally because the ioreq state will always be
STATE_IOREQ_NONE at the end of hvm_io_assist() so the test was
pointless. These calls are also only relevant when the emulation has
been handled externally which is now always the case.

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