Don't include control characters in BufferIn trace output (RHBZ#646822).

Don't include control characters in BufferIn trace output (RHBZ#646822).

The example below shows what the output looks like for a large
random buffer.

$ guestfish -N fs -m /dev/sda1 -x -- \
    touch /test : \
    pwrite /test "$(dd if=/dev/urandom bs=128k count=1)" 0
[...]
pwrite "/test" "\x7f\xa0/\xb3\x80\xd3\xbc\xc3\xc3.\xb1\xe0\x1b\xafC\x06\xd5;\x0ajJ[o\xc1\xdd\xae\x1f\xce\xb2\x8d\xb3\xd0\x11\xcc$%\xe6<\xc7\xc7\xe7BU*\xc4l%\xaa\xea\xe9\x1an\xda]\xc6I\x0eC\xf9;\xec\x12a\x1f\xeaRH\xb2P\xd6+\xc4\xe6\xa5bW\x99\\x9d\xc8\x9bJ\xef\x99-\x16:h5\xe2\x0f\xa2\xa08\x9bU\x0b$\x138\xcf\xd4j\x9b\x83{%\xac0\xdaa1Xx\xbd`\x8e\xdd\x82\x87\x07\x98\xd2\x9ed\x8bq\xd0\x1f5\x8f\xab\xad4z1\xda\xc4b\xc1\xbc\x0f\xaa\xea\xc1\x15(\xfd1\xc2\x0bF\xe6\x9e\xb0+/g\\xab\xb0b\xde_\xca\xf9\xad\xe1?%\x17\xad\x98\xa4e\xc1\xe0f'\x89\xe9>\xff\xadhYi\xe7\x8c]%\xef\xe0\xa1R\xe5\xd5\x03K\xefI\xdf\xad\xd3\x82\xdb\x0f\xdd\xc3\x8f"\xf1G\xea\xf9r\xdd\xff\x88\x81\xb7\xf2\x0e\x0f\x1d;:\xf2F1\xdb\xb5D\xa1^\x928\xf5\x8e)\xab\xc4\xc3H(\xd0ol\xc6\xe4\xd6\xa3L\x1c\x06\xf4"<truncated, original size 130567 bytes> 0
[...]