vmm: Convert VM_MAXCPU into a loader tunable hw.vmm.maxcpu.

The default is now the number of physical CPUs in the system rather
than 16.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37175

(cherry picked from commit ee98f99d7a68b284a669fefb969cbfc31df2d0ab)