xen: move xc_interface compatibility fallback further up the file

xen: move xc_interface compatibility fallback further up the file

We are going to want to use the dummy xendevicemodel_handle type in
new stub functions in the CONFIG_XEN_CTRL_INTERFACE_VERSION < 41000
section.  So we need to provide that definition, or (as applicable)
include the appropriate header, earlier in the file.

(Ideally the newer compatibility layers would be at the bottom of the
file, so that they can naturally benefit from the compatibility layers
for earlier version.  But that's rather too much for this series.)

No functional change.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>