libvmmapi: Add vm_close()

Currently there is no way to safely free a vm structure without
leaking the fd. vm_destroy() closes the fd but also destroys the VM
whereas in some cases a VM needs to be opened (vm_open) and then
closed (vm_close).

Reviewed by: jhb
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D35073