libvmmapi: Use the vmmctl device file to create and destroy VMs

This deprecates the vm_create() and vm_open() interfaces and introduces
vm_openf(), which takes flags controlling its behaviour. In particular,
it will optionally create a VM first, and it can optionally reinitialize
an existing VM. This enables some simplification of existing consumers.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D47030