xbzrle.c: Avoid undefined behaviour with signed arithmetic

Use unsigned types for doing bitwise arithmetic in the xzbrle
calculations, to avoid undefined behaviour:

 xbzrle.c:99:49: runtime error: left shift of 72340172838076673
 by 7 places cannot be represented in type 'long'

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
