engine: Split update of 'isPlugged' and 'bootOrder' properties...

HotPlugDiskToVmCommand updates 'isPlugged' and 'bootOrder' of all
devices attached to any particular VM. It should only update 'isPlugged'
for the device that was plugged by this command, otherwise it can cause a
race with another thread which handles the hot plug of another disk for
the same VM.

Change-Id: I1359a34a48a6261e22631ff1640d81d735e8c490
Bug-Url: https://bugzilla.redhat.com/1003649
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>