vmm: Use vm_get_maxcpus() instead of VM_MAXCPU in various places.

Mostly these are loops that iterate over all possible vCPU IDs for a
specific virtual machine.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37147

(cherry picked from commit 35abc6c238e98e313c5b1cb5ed18b8ed68615abc)