virt/kvm: Replace spin_is_locked() with lockdep

virt/kvm: Replace spin_is_locked() with lockdep

lockdep_assert_held() is better suited to checking locking requirements,
since it only checks if the current thread holds the lock regardless of
whether someone else does. This is also a step towards possibly removing
spin_is_locked().

Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: "Radim Krčmář" <[email protected]>
Cc: <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>