ioreq-server: make buffered ioreq handling optional

ioreq-server: make buffered ioreq handling optional

Some emulators will only register regions that require non-buffered
access. (In practice the only region that a guest uses buffered access
for today is the VGA aperture from 0xa0000-0xbffff). This patch therefore
makes allocation of the buffered ioreq page and event channel optional for
secondary ioreq servers.

If a guest attempts buffered access to an ioreq server that does not
support it, the access will be handled via the normal synchronous path.

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