Allow custom name for disk device

This is needed for example for xen in full virtualisation mode
as it won't allow the default vda

lib/vagrant-libvirt/action/create_domain.rb

@@ -42,6 +42,7 @@ module VagrantPlugins
           @machine_type = config.machine_type
           @machine_arch = config.machine_arch
           @disk_bus = config.disk_bus
+          @disk_device = config.disk_device
           @nested = config.nested
           @memory_size = config.memory.to_i * 1024
           @management_network_mac = config.management_network_mac

lib/vagrant-libvirt/config.rb

@@ -71,6 +71,7 @@ module VagrantPlugins
       attr_accessor :machine_arch
       attr_accessor :machine_virtual_size
       attr_accessor :disk_bus
+      attr_accessor :disk_device
       attr_accessor :nic_model_type
       attr_accessor :nested
       attr_accessor :volume_cache
@@ -170,6 +171,7 @@ module VagrantPlugins
         @machine_arch      = UNSET_VALUE
         @machine_virtual_size = UNSET_VALUE
         @disk_bus          = UNSET_VALUE
+        @disk_device       = UNSET_VALUE
         @nic_model_type    = UNSET_VALUE
         @nested            = UNSET_VALUE
         @volume_cache      = UNSET_VALUE
@@ -569,6 +571,7 @@ module VagrantPlugins
         @machine_arch = nil if @machine_arch == UNSET_VALUE
         @machine_virtual_size = nil if @machine_virtual_size == UNSET_VALUE
         @disk_bus = 'virtio' if @disk_bus == UNSET_VALUE
+        @disk_device = 'vda' if @disk_device == UNSET_VALUE
         @nic_model_type = 'virtio' if @nic_model_type == UNSET_VALUE
         @nested = false if @nested == UNSET_VALUE
         @volume_cache = 'default' if @volume_cache == UNSET_VALUE

lib/vagrant-libvirt/templates/domain.xml.erb

@@ -74,7 +74,7 @@
       <driver name='qemu' type='qcow2' cache='<%= @domain_volume_cache %>'/>
       <source file='<%= @domain_volume_path %>'/>
       <%# we need to ensure a unique target dev -%>
-      <target dev='vda' bus='<%= @disk_bus %>'/>
+      <target dev='<%= @disk_device %>' bus='<%= @disk_bus %>'/>
     </disk>
     <% end %>
 <%# additional disks -%>