vmm(4): reference count vm's and vcpu's

vmm(4): reference count vm's and vcpu's

Unlocking most of vmm last year at k2k21 exposed bugs related to
lifetime management of vm and vcpu objects.

Add reference counts to make sure we don't attempt to teardown vcpu
or vm related objects while a thread is holding a reference. This
also reduces abuse of rwlocks originally intended to protect the
linked lists cleaning things up quite a bit. While here, also
document assumptions on how struct members are protected for the
next brave soul wander in.

ok mlarkin@