KVM: x86/mmu: change TDP MMU yield function returns to match cond_resched

Currently the TDP MMU yield / cond_resched functions either return
nothing or return true if the TLBs were not flushed. These are confusing
semantics, especially when making control flow decisions in calling
functions.

To clean things up, change both functions to have the same
return value semantics as cond_resched: true if the thread yielded,
false if it did not. If the function yielded in the _flush_ version,
then the TLBs will have been flushed.

Reviewed-by: Peter Feiner <pfeiner@google.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210202185734.1680553-2-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>