Allow creation of LXC with bdevspecs.

New Container#create accept 5 parameters:
container.create(template, bdevtype = nil, bdevspecs = {}, flags = 0, args = [])

Example:

myspec = { fstype: 'ext4', fssize: 32212254720, vgname: 'zephirworks_VG'}

c = LXC::Container.new('zephirworks_example')
c.create("ubuntu", "lvm", myspec)