ioreq-server: add support for multiple servers

ioreq-server: add support for multiple servers

The previous single ioreq server that was created on demand now
becomes the default server and an API is created to allow secondary
servers, which handle specific IO ranges or PCI devices, to be added.

When the guest issues an IO the list of secondary servers is checked
for a matching IO range or PCI device. If none is found then the IO
is passed to the default server.

Secondary servers use guest pages to communicate with emulators, in
the same way as the default server. These pages need to be in the
guest physmap otherwise there is no suitable reference that can be
queried by an emulator in order to map them. Therefore a pool of
pages in the current E820 reserved region, just below the special
pages is used. Secondary servers allocate from and free to this pool
as they are created and destroyed.

The size of the pool is currently hardcoded in the domain build at a
value of 8. This should be sufficient for now and both the location and
size of the pool can be modified in future without any need to change the
API.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Fix build errors in xen/xsm/dummy.c and xen/xsm/flask/hooks.c with XSM
enabled.

Signed-off-by: Jan Beulich <jbeulich@suse.com>