emulators/virtualbox-ose-kmod: Add check for MAXCPU and mp_maxcpus before load vboxdrv.ko

If the MAXCPU option is not the default for the running kernel, then
'kldload vboxdrv.ko' will result in a kernel panic.
Due to ABI difference (vboxdrv passes cpuset_t parameter (bitfield with
CPU_SETSIZE -> MAXCPU bits) into smp_rendezvous_cpus()) kernel panics
with "ncpus is 0 with non-zero map" message.

PR: 219913