Fix accounting related to addition of md member devices.

We should not be incrementing memberDevices if we're just adding a member
device as part of devicetree.populate. In that case we already know the
number of memberDevices and shouldn't be changing it. It should be
incremented when adding a new member to an existing array, like when
scheduling a ContainerAddMember action.
Pass UUID of existing md array to superclass constructor.
Use the right md UUID when trying to look one up from addUdevDevice.

In this case we want the MD UUID -- not the filesystem UUID. The MD
array always has a UUID, but it may or may not contain any formatting
that has its own UUID. Using the MD UUID is therefore more reliable.
Tell lvm to prefer /dev/mapper/ and /dev/md/ to dm-X and mdX nodes.
Move changes from action ctors into apply methods.

The apply method is called from DeviceTree.registerAction, so there is no
need to change any code that creates actions.