Use hio_ prefix for handle I/O calls

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix build of multi-file loaders

Multi-file loaders must handle memory I/O wrappers in a special way
because extra files are always read from files.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Fix sample loading tests to use handle I/O

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