(#20915) Read dmi entries as binary data

The DMI entry information that is read to determine the virtual platform is
actually binary data. When it is interpreted as an encoded string, then there
will be encoding errors. This changes it to be read in "rb" mode, which has
nearly no effect on ruby 1.8.7, but causes it to be read as
Encoding::ASCII_8BIT on ruby 1.9.3 (which is how ruby deals with binary
data).