Split out from validate_prot_to_pageflags, as there is not
one single host_prot for the entire range.  We need to adjust
prot for every host page that overlaps multiple guest pages.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230707204054.8792-13-richard.henderson@linaro.org>