xbzrle: fix compilation on ppc32

When compiling the xbzrle code on my ppc32 user space, I hit the following
gcc compiler warning (treated as an error):

  cc1: warnings being treated as errors
  savevm.c: In function ‘xbzrle_encode_buffer’:
  savevm.c:2476: error: overflow in implicit constant conversion

Fix this by making the cast explicit, rather than implicit.

Signed-off-by: Alexander Graf <agraf@suse.de>
