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>