[Installer]: [FEAT] try harder to umount
[Installer]: [FEAT] try harder to umount

Sometimes, kpartx -d for freeing an image fails even though the mount is
no longer busy. In that case we give kpartx another chance to free the
image after some time for all relevant buffers to settle. The issue
occured with SimNow since SimNow tests don't start with a reboot and
thus the machine will eventually run out of free loopback devices when
kpartx doesn't free its image.

tapper/Tapper-Installer,tapper/Tapper-Installer