honour path parameter of storage directive

Fix use of the `:path` parameter to the `storage` directive:

    config.vm.provider :libvirt do |libvirt|
      libvirt.storage :file, :path => 'my-disk.qcow2'
    end

The value of the `:path` parameter was being correctly used in
`domain.xml.erb` for defining the VM, but it was not passed to libvirt
via fog's volume creation mechanism:

    lib/fog/libvirt/models/compute/templates/volume.xml.erb

Instead, libvirt determines the backing file from the `<name>` element,
so we ensure that the `:name` parameter contains the correct filename.
Merge branch 'master' into bp-disk-path
Check for absolute disk path in #validate
Do not chance disk path

Mutating path caused vagrant to constatnyl spew errors from #validate
while waiting for ssh to be available. Instead, create a new key.
Merge pull request #275 from sciurus/disk-path

honour path parameter of storage directive and handle errors