I had the same problem, this is how I make it work:
-Edit the final path (the decrypted path) “/run/mount/user/decrypt/Whonix-*.qcow2”
EDITOR=vi virsh edit Whonix-Workstation
EDITOR=vi virsh edit Whonix-Gateway
-Now decrypt the images and set permissions:
cryptsetup open --type tcrypt /run/mount/user/usb/encrypteddisk decrypted && chmod +x /run/mount/user
And thats all
Cheers