If needs to be extended - do it before import. If needs to be reduced -
after. This way, if data import fails for any reason, previous data
won't be destroyed (truncated).
Also, convert error on shrinking volume to a warning, as it doesn't break
the template (just leave it with bigger disk than needed). Currently all
storage pool implementations refuse to shrink a volume (but it may
change in the future).
QubesOS/qubes-issues#3169
Do not close event loop in utility function - handle it only in main().
For this reason, change appropriate functions to coroutines.
FixesQubesOS/qubes-issues#2865
Enable 'qrexec' VM feature to wait for qrexec initialization - it is
required to call qubes.PostInstall service. If VM start fails, assume
there is no qrexec and drop that feature.
