Or you can do this within Qubes:
1. Interrupt the boot process and change the boot parameters to add rd.break=cleanup.
2. Boot.
You'll be prompted to decrypt disks and then drop to shell.
The root filesystem will be mounted ro at /sysroot.
3. umount /sysroot.
Mount it rw somewhere else (e.g /mnt - you'll have to create that)
4. mkdir /mnt
5. mount /dev/mapper/blah /mnt
6. cd /mnt/var/lib/qubes/servicevms
7. mv sys-usb sys-usb.bak
8. umount /mnt
Remount /dev/mapper/blah on /sysroot
9. mount /dev/mapper/blah /sysroot
10. exit
exit from the rescue shell
The system should then continue to boot - sys-usb wont be able to start.
Once up, 'mv sys-usb.bak sys-usb', and then fix whatever issue is causing
the crash, or remove the qube and recreate it.