KVM: selftests: Close VM's binary stats FD when releasing VM

KVM: selftests: Close VM's binary stats FD when releasing VM

Close/free a VM's binary stats cache when the VM is released, not when the
VM is fully freed.  When a VM is re-created, e.g. for state save/restore
tests, the stats FD and descriptor points at the old, defunct VM.  The FD
is still valid, in that the underlying stats file won't be freed until the
FD is closed, but reading stats will always pull information from the old
VM.

Note, this is a benign bug in the current code base as none of the tests
that recreate VMs use binary stats.

Fixes: 83f6e109f562 ("KVM: selftests: Cache binary stats metadata for duration of test")
Link: https://lore.kernel.org/r/20250111005049.1247555-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>