As part of plumbing MemTxAttrs down to the IOMMU translate method,
add MemTxAttrs as an argument to memory_region_access_valid().
Its callers either have an attrs value to hand, or don't care
and can use MEMTXATTRS_UNSPECIFIED.

The callsite in flatview_access_valid() is part of a recursive
loop flatview_access_valid() -> memory_region_access_valid() ->
 subpage_accepts() -> flatview_access_valid(); we make it pass
MEMTXATTRS_UNSPECIFIED for now, until the next several commits
have plumbed an attrs parameter through the rest of the loop
and we can add an attrs parameter to flatview_access_valid().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180521140402.23318-8-peter.maydell@linaro.org