gdbstub: Fix handle_query_xfer_auxv

gdbstub: Fix handle_query_xfer_auxv

The main problem was that we were treating a guest address
as a host address with a mere cast.

Use the correct interface for accessing guest memory.  Do not
allow offset == auxv_len, which would result in an empty packet.

Fixes: 51c623b0de1 ("gdbstub: add support to Xfer:auxv:read: packet")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210128201831.534033-1-richard.henderson@linaro.org>
Message-Id: <20210202134001.25738-11-alex.bennee@linaro.org>