memory: limit sections in the radix tree to the actual address space size

The radix tree is statically sized to fit TARGET_PHYS_ADDR_SPACE_BITS.
If a larger memory region is registered, it will overflow.

Fix by limiting any section in the radix tree to the supported size.

This problem was not observed earlier since artificial regions (containers
and aliases) are eliminated by the memory core, leaving only device regions
which have reasonable sizes.  An IOMMU however cannot be eliminated by the
memory core, and may have an artificial size.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
[ Fail the build if TARGET_PHYS_ADDR_SPACE_BITS is too large - Paolo ]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>