ioreq-server: centralize access to ioreq structures

ioreq-server: centralize access to ioreq structures

To simplify creation of the ioreq server abstraction in a subsequent patch,
this patch centralizes all use of the shared ioreq structure and the
buffered ioreq ring to the source module xen/arch/x86/hvm/hvm.c.

The patch moves an rmb() from inside hvm_io_assist() to hvm_do_resume()
because the former may now be passed a data structure on stack, in which
case the barrier is unnecessary.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>