Change memory i/o access to allow us to set a file size

When opening a file from memory, add the option to set a file size. This
will allow us to test some file formats and compute the MD5 digest of a
file in memory.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Update tests to use the new hio API

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>