I haven't been able to track down any rhyme or reason for its removal but lomount is not available in the xen-utils package for Debian.

The same functionality can at least be achieved, though in a more round-about way, with kpartx:

# kpartx -l router.hdd 
loop3p1 : 0 4192964 /dev/loop3 1
# kpartx -a router.hdd
# mount /dev/mapper/loop3p1 /mnt/tmp