x86/hvm: use ioreq_t to track in-flight state

x86/hvm: use ioreq_t to track in-flight state

Use an ioreq_t rather than open coded state, size, dir and data fields
in struct hvm_vcpu_io. This also allows PIO completion to be handled
similarly to MMIO completion by re-issuing the handle_pio() call.

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