KVM: VMX: Tell the nested hypervisor to skip L1D flush on vmentry

KVM: VMX: Tell the nested hypervisor to skip L1D flush on vmentry

When nested virtualization is in use, VMENTER operations from the nested
hypervisor into the nested guest will always be processed by the bare metal
hypervisor, and KVM's "conditional cache flushes" mode in particular does a
flush on nested vmentry.  Therefore, include the "skip L1D flush on
vmentry" bit in KVM's suggested ARCH_CAPABILITIES setting.

Add the relevant Documentation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Orabug: 28220674
CVE: CVE-2018-3646

(cherry picked from commit 5b76a3cff011df2dcb6186c965a2e4d809a05ad4)

Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Conflicts:
	arch/x86/kvm/vmx.c
Contextual: we did not have "if(boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))"