core: Streamline MoveVmCommand.canDoAction

MoveVmCommand's constructor uses getVm().getStoragePoolId(), so it is
safe to assume that the VM is not null in the canDoAction() method.

This method removes this redundant check from canDoAction(), and moves
all the initializer logic from it to the constructor, where it belongs.

Change-Id: I047f744cdaf8f7b1b5a788c9435642eb05ed6919
Signed-off-by: Allon Mureinik <amureini@redhat.com>