If you use file-based guest images, one may have increased the number of configured loop devices (the default allows up to 8 loop devices to become active). If you need more than 8 file-based guests/loop devices, you must modify the /etc/modprobe.conf file. When modifying the modprobe.conf file, you must include this line:

options loop max_loop=64

You can substitute the number 64 for another number that corresponds to your configuration. Note that it might not be ideal to employ loop device backed guests on your system. Instead you can use phy: block device or tap:aio file backed devices on paravirtual systems, and phy: device backed or file: file backed for full virtualization.