Canonicalize distro & osfamily params in os_cls()

If the distro name as returned by libguestfs's inspect_get_distro was
redhat-based, since hyphens are not allowed in module names, importing
module redhat-based would create an ImportError exception, even if file
redhat-based.py was present. Although we don't have any redhat-based.py
module and the code will still correctly fall back to importing
linux.py this can be considered as a bug. In order to overcome this, we
replace the hyphens with underscores in the distro and osfamily names.
In the future, if we have to create a module for redhat-based distros we
will name it redhat_based.py.
Correctly identify Oracle Linux OSes

libguestfs will identify Oracle Linux as Red Hat Enterprise Linux. Check
for the presence of /etc/oracle-linux to determine if the OS is Oracle
Linux.