target/arm/ptw: Check for block descriptors at invalid levels

target/arm/ptw: Check for block descriptors at invalid levels

The architecture doesn't permit block descriptors at any arbitrary
level of the page table walk; it depends on the granule size which
levels are permitted.  We implemented only a partial version of this
check which assumes that block descriptors are valid at all levels
except level 3, which meant that we wouldn't deliver the Translation
fault for all cases of this sort of guest page table error.

Implement the logic corresponding to the pseudocode
AArch64.DecodeDescriptorType() and AArch64.BlockDescSupported().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230807141514.19075-14-peter.maydell@linaro.org