node_device: Don't leak error message buffer from virMdevctlListDefined|Active

node_device: Don't leak error message buffer from virMdevctlListDefined|Active

nodeDeviceUpdateMediatedDevices invokes virMdevctlListDefined and
virMdevctlListActive both of which were passed the same 'errmsg' buffer.

Since virCommandSetErrorBuffer() always allocates the error buffer one
of them was leaked.

Fix it by populating the 'errmsg' buffer only on failure of
virMdevctlListActive|Defined which invoke the command.

Add a comment to nodeDeviceGetMdevctlListCommand reminding how
virCommandSetErrorBuffer() works.

Fixes: 44a0f2f0c8f
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>