added bdevspec param to LXC::Controller#create

Hi guys,

I added to LXC::Controller#create the bdev_spec params which allow to use bdev_type like 'lvm' or 'zfs'.

I didn't figure out how to make a test case on this basically because the tests needs an volume group (with freespace) or a zfs filesystem but I'm keen to get your idea about testing.

My C skills are a little bit rusty so review carefully the code before merging.

Cheers,
Andrea.