x86/hvm/ioreq: simplify code and use consistent naming

x86/hvm/ioreq: simplify code and use consistent naming

This patch re-works much of the ioreq server initialization and teardown
code:

- The hvm_map/unmap_ioreq_gfn() functions are expanded to call through
  to hvm_alloc/free_ioreq_gfn() rather than expecting them to be called
  separately by outer functions.
- Several functions now test the validity of the hvm_ioreq_page gfn value
  to determine whether they need to act. This means can be safely called
  for the bufioreq page even when it is not used.
- hvm_add/remove_ioreq_gfn() simply return in the case of the default
  IOREQ server so callers no longer need to test before calling.
- hvm_ioreq_server_setup_pages() is renamed to hvm_ioreq_server_map_pages()
  to mirror the existing hvm_ioreq_server_unmap_pages().

All of this significantly shortens the code.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>