Prevent concurrent qvm-sync-clock calls
Prevent concurrent qvm-sync-clock calls

In some cases qvm-sync-clock can take a long time (for example in case
of network problems, or when some do not responds). This can lead to
multiple qvm-sync-clock hanging for the same reason (blocking vchan
resources). To prevent that create a lock file and simply abort when one
instance is already running.

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin