util/hexdump: introduce qemu_hexdump_line()

Dumping one line of hexadecimal/ASCII from a buffer is often needed.
Move this part from qemu_hexdump() and use it

Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20200925091055.186023-2-lvivier@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>