I have tried to mount a virtual disk image on my Fedora host using the command
qemu-nbd -c /dev/ndb0 rawhide.img
and then detecting the LVM logical volumes and mounting the one I wanted. Obviously, the virtual machine is not running - I'm not stupid. However, because I have found this to be unreliable (nbd dies after a while), and also slow, I have decided to just copy the filesystem into a subdirectory of my home directory:
cp -a /mnt/rawhide .
However, I can't even get nbd to stay alive long enough to do the copy! It just died after less than one minute!
I can't use the fast offset= technique (at least, not as stated) from this page because the filesystem is inside LVM on the virtual disk image. I think there was some utility to calculate the offset for you, but I can't remember what it was called, or whether it works with LVM?