launch: direct: Only issue Debian /dev/kvm group warning if /dev/kvm wasn't openable (RHBZ#1130189).

This warning was meant for the case where /dev/kvm is inaccessible and
the user must add themselves to a special group.

However we didn't take into account whether /dev/kvm was openable in
this test.  If it's openable, no point issuing the warning.

Move the is_openable ("/dev/kvm") test earlier, and don't issue the
warning if that was successful.