Instance failover: fix bug for INT_MIRROR cases

Patches db366d9a and aac4511a added support for EXT_MIRROR instances,
but inadvertently introduced a bug: for INT_MIRROR cases, we don't
need (actually we can't support) neither an iallocator nor a target
node.

To fix this, we move the iallocator/node checks in CheckPrereq (or
respectively in the tasklet CheckPrereq), where we have access to the
instance configuration, and additionally we check for and prevent
passing either of these two for INT_MIRROR instances.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>