accel/tcg: Avoid abi_ptr in user-exec.c

accel/tcg: Avoid abi_ptr in user-exec.c

In page_dump/dump_region, use guest_addr_max to check the
size of the guest address space and size the output
appropriately.  This will change output with small values
of -R reserved_va, but shouldn't affect anything else.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>