xen: move xen_sysdev to xen_backend.c

Commit 9432e53a added xen_sysdev as a
system device to serve as an anchor for removable virtual buses. This
introduced a build failure for non-x86 builds with CONFIG_XEN_BACKEND
set, as xen_sysdev was defined in a x86 specific file while being
consumed in an architecture independent source.

Move the xen_sysdev definition and initialization to xen_backend.c to
avoid the build failure.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>