vmm: Remove an unneeded lookup in devmem_create_cdev()

The sole caller of this function already holds a pointer to the VM's
softc, so rather than passing the VM name and looking it up again, just
pass the softc pointer directly. This function is only called from an
ioctl context, so the softc structure will remain live.

Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D46446