g_io_error_from_errno returns a GIOErrorEnum, and vala doesn't know
it should convert it to a GIOError *, which causes compilation issues
with:

// valac-0.16 --pkg gio-2.0 foo.vala
// /home/teuf/jhbuild-boxes/sources/gnome-boxes/foo.vala.c: In function ‘test’:
// /home/teuf/jhbuild-boxes/sources/gnome-boxes/foo.vala.c:26:9: erreur:
// incompatible types when assigning to type ‘struct GError *’ from type ‘GIOErrorEnum’
// error: cc exited with status 26
// Compilation failed: 1 error(s), 0 warning(s)

void test () throws GLib.Error {
    throw GLib.IOError.from_errno(22); // EINVAL
}

int main () {
    try {
        test ();
    } catch (GLib.Error e) {}

    return 0;
}

This is vala bug 664530. This workaround was suggested by vala
maintainers as the bug isn't trivial to fix.

https://bugzilla.gnome.org/show_bug.cgi?id=679752