libvmm: constify vm_get_name()

Allows callers of vm_get_name() to retrieve the vm name without having
to allocate a buffer.

While in the vicinity, do minor cleanup in vm_snapshot_basic_metadata().

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D34290

(cherry picked from commit 3efc45f34e13c63cb37a77a67fea39937004b75a)