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# First map the loop device to your KVM disk image.
$ losetup -fv /kvm/watercooler.mwt2.org.img 
Loop device is /dev/loop0
 
# No partitions!    
$ ls /dev/loop0* /dev/mapper/loop0*
ls: /dev/mapper/loop0*: No such file or directory
/dev/loop0
 
# Use kpartx -a to discover and device-map the partitions.
$ kpartx -av /dev/loop0
add map loop0p1 : 0 204800 linear /dev/loop0 2048
add map loop0p2 : 0 16930816 linear /dev/loop0 206848
add map loop0p3 : 0 16416768 linear /dev/loop0 17137664
 
# Look, partitions.
$ ls /dev/loop0* /dev/mapper/loop0*
/dev/loop0  /dev/mapper/loop0p1  /dev/mapper/loop0p2  /dev/mapper/loop0p3
 
# Fiddle with /dev/loop0p1 et al.
$ mount /dev/mapper/loop0p1 /tmp/myfolder
$ umount /tmp/myfolder
 
# Try to un-loop the file.  You can't, because kpartx's partition devices are still mapped.
$ losetup -d /dev/loop0
ioctl: LOOP_CLR_FD: Device or resource busy
 
# Ask kpartx to unmap.
$ kpartx -dv /dev/loop0
del devmap : loop0p1
del devmap : loop0p2
del devmap : loop0p3
 
# Now you can un-loop.
$ losetup -d /dev/loop0