units: Workaround for user-configvirtfs.service

As Jeremi noticed with the current Flatcar Stable 3510.2.7 and a custom
Docker brought in as systemd-sysext image, the qemu VM didn't pick up
the SSH keys anymore. This couldn't be reproduced with Alpha, though.
The culprit is not completely known but it seems to be a race condition
that makes the Wants= dependency of media-configvirtfs.mount not take
action.
To work around the problem, use Upholds= which eventually starts
user-configvirtfs.service.

units/media-configvirtfs.mount

@@ -1,5 +1,5 @@
 [Unit]
-Wants=user-configvirtfs.service
+Upholds=user-configvirtfs.service
 Before=user-configvirtfs.service
 # Only mount config drive block devices automatically in virtual machines
 # or any host that has it explicitly enabled and not explicitly disabled.