Mac: Try to work around the re-occurring disk image creation error

A guess is that hdiutil finishes even though the system is not yet quite
finished with the image (e.g. still unmounting?), so introducing a small
timer.
Also turn on verbosity, so this error might be better trackable if it
occurs again.

Change-Id: I0acb23bbac408f2979c8cacf2fc58fdff782fc3d
Reviewed-by: Eike Ziller <eike.ziller@digia.com>