src: stop using O_DIRECTORY in resctrl

src: stop using O_DIRECTORY in resctrl

The O_DIRECTORY flag causes open() to return an error
if the filename is a directory. There's no obvious
reason why resctrl needs to use this, while the rest of
libvirt code does not. Removing it avoids build issues
on platforms where O_DIRECTORY is not defined, once we
remove GNULIB.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>