util/hexdump: Use a GString for qemu_hexdump_line

Allocate a new, or append to an existing GString instead of
using a fixed sized buffer.  Require the caller to determine
the length of the line -- do not bound len here.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240412073346.458116-4-richard.henderson@linaro.org>