KVM: selftests: Get VM's binary stats FD when opening VM

KVM: selftests: Get VM's binary stats FD when opening VM

Get and cache a VM's binary stats FD when the VM is opened, as opposed to
waiting until the stats are first used.  Opening the stats FD outside of
__vm_get_stat() will allow converting it to a scope-agnostic helper.

Note, this doesn't interfere with kvm_binary_stats_test's testcase that
verifies a stats FD can be used after its own VM's FD is closed, as the
cached FD is also closed during kvm_vm_free().

Link: https://lore.kernel.org/r/20250111005049.1247555-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>