kernel > 2.6.26
modprobe nbd max_part=8

qemu-nbd root.qcow2

qemu-nbd --connect=/dev/nbd0 root.qcow2

At which point you can view the disk partitions:

fdisk /dev/nbd0

or mount a disk, such as

mount /dev/nbd0p1 /mnt

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prior kernels:

modprobe nbd  (without arguments)
use info from mount_disk_image