nvmx: mark ept single context invalidation as supported

Commit 4b855078 ("KVM: nVMX: Don't advertise single
context invalidation for invept") removed advertising
single context invalidation since the spec does not mandate it.
However, some hypervisors (such as ESX) require it to be present
before willing to use ept in a nested environment. Advertise it
and fallback to the global case.

Signed-off-by: Bandan Das <bsd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>