core: programatically inject backend into Commands

Commands, specially storage commands must have backend instance in the
constructor.

- example CloneVmCommand constructor is running internalQuery

This contradicts with DI as dependincies can be injected only after you
have and instance in hand.

So we're still can't use proper Injection of dependencies into commands.

the real fix is to move parts that need dependencies into @PostConstruct
methods.

Change-Id: I38d2aa7f14f13cb71a00f9080e3ec97d8884eea3
Signed-off-by: Roy Golan <rgolan@redhat.com>