KVM: nVMX: initialize vmcs02 constant exactly once (per VMCS)

Add a dedicated flag to track if vmcs02 has been initialized, i.e.
the constant state for vmcs02 has been written to the backing VMCS.
The launched flag (in struct loaded_vmcs) gets cleared on logical
CPU migration to mirror hardware behavior[1], i.e. using the launched
flag to determine whether or not vmcs02 constant state needs to be
initialized results in unnecessarily re-initializing the VMCS when
migrating between logical CPUS.

[1] The active VMCS needs to be VMCLEARed before it can be migrated
    to a different logical CPU.  Hardware's VMCS cache is per-CPU
    and is not coherent between CPUs.  VMCLEAR flushes the cache so
    that any dirty data is written back to memory.  A side effect
    of VMCLEAR is that it also clears the VMCS's internal launch
    flag, which KVM must mirror because VMRESUME must be used to
    run a previously launched VMCS.

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>