ioreq-server: create basic ioreq server abstraction

ioreq-server: create basic ioreq server abstraction

Collect together data structures concerning device emulation together into
a new struct hvm_ioreq_server.

Code that deals with the shared and buffered ioreq pages is extracted from
functions such as hvm_domain_initialise, hvm_vcpu_initialise and do_hvm_op
and consolidated into a set of hvm_ioreq_server manipulation functions. The
lock in the hvm_ioreq_page served two different purposes and has been
replaced by separate locks in the hvm_ioreq_server structure.

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