nestedhvm: Remove nhvm_{initialise,destroy,reset}.

nestedhvm: Remove nhvm_{initialise,destroy,reset}.

They are a pointless level of abstraction beneath nestedhvm_* variants
of the same operations, which all callers should be using.

At the same time, nestedhvm_vcpu_initialise() does not need to call
destroy if initialisation fails. That is the vendor-specific init
function's job (clearing up its own state on failure).

Signed-off-by: Keir Fraser <keir@xen.org>