xen: make use of xen_xc implicit in xen_common.h inlines

xen: make use of xen_xc implicit in xen_common.h inlines

Doing this will make the transition to using the new libxendevicemodel
interface less intrusive on the callers of these functions, since using
the new library will require a change of handle.

NOTE: The patch also moves the 'externs' for xen_xc and xen_fmem from
      xen_backend.h to xen_common.h, and the declarations from
      xen_backend.c to xen-common.c, which is where they belong.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>