core: Add storage validation when plugging an image.

When a plugging a disk to a VM, the engine should validate that the storage
domain that the disk resides on is active.
There is no reason to call VDSM if it can't connect to the storage.

The validation should be done on plug disk, and also on its derived
operation attachDisk.

Change-Id: Ieeb2522bc5aa280b9b98ef728737aeeaa82d1263
Bug-Url: https://bugzilla.redhat.com/969767
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
core: extract method to ImagesHandler

Add a method which returns the activated volume from the image volumes.

Change-Id: I7700c28323908128a63910866eddb7dfa4b112fb
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
core: Enhance for loop at import VM.

The for loop in the import VM commands iterates over the volumes of
the image, besides the last one.
this behaviour is not logic and error prone, all volumes of the image
should be set with the same disk id

Change-Id: I6f6ca389739143f5102e0ef940fe6fac0164b17d
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>