Check for empty result from fog-libvirt volumes

The fog-libvirt volumes filter behaves differently than the others in
that it returns an empty volume object if nothing is found. This has the
potential to change in the future so guard against an empty result as
well by checking if the assigned first element is nil.

lib/vagrant-libvirt/action/create_domain_volume.rb

@@ -28,7 +28,7 @@ module VagrantPlugins
           domain_volume = env[:machine].provider.driver.connection.volumes.all(
             name: @name
           ).first
-          raise Errors::DomainVolumeExists if domain_volume.id
+          raise Errors::DomainVolumeExists if domain_volume && domain_volume.id
 
           # Get path to backing image - box volume.
           box_volume = env[:machine].provider.driver.connection.volumes.all(

lib/vagrant-libvirt/action/handle_box_image.rb

@@ -63,9 +63,10 @@ module VagrantPlugins
           # locking all subsequent actions as well.
           @@lock.synchronize do
             # Don't continue if image already exists in storage pool.
-            break if env[:machine].provider.driver.connection.volumes.all(
+            box_volume = env[:machine].provider.driver.connection.volumes.all(
               name: env[:box_volume_name]
-            ).first.id
+            ).first
+            break if box_volume && box_volume.id
 
             # Box is not available as a storage pool volume. Create and upload
             # it as a copy of local box image.