vmm: Postpone vmm module initialization to after SI_SUB_DEVFS

vmmops_modinit() needs to create a device file, and this must happen
after SI_SUB_DEVFS. On non-EARLY_AP_STARTUP platforms (i.e., !x86) this
happens already by accident, but we shouldn't rely on it.

On riscv, remove the current SI_SUB_SMP ordering since that was copied
from arm64 and isn't needed. In particular, riscv's vmmops_modinit()
does not call smp_rendezvous().

Reported by: Oleksandr Kryvulia <shuriku@shurik.kiev.ua>
Fixes: a97f683fe3c4 ("vmm: Add a device file interface for creating and destroying VMs")