source

# First check if there is already a loop
losetup -a

# get the next loop device available
losetup -f

# 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

# 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