Add a CheckNodeVmCapable helper in cmdlib

Also changes the error code for the other CheckNode* helpers to
ECODE_STATE, not ECODE_INVAL: ECODE_INVAL is for requests that are
invalid (e.g. create drbd instance with one node), whereas ECODE_STATE
denote requests that are not satisfiable due to cluster/node/instance
state.

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