Add disk property 'shareable'

To simulate shared SAN storage an additional libvirt
disk property needs to be set which disables caching.

Also updated the documentation to document this new
property and added an example on how to simulate
shared SAN storage

Closes #648

README.md

@@ -591,9 +591,9 @@ It has a number of options:
   [here](https://www.suse.com/documentation/sles11/book_kvm/data/sect1_chapter_book_kvm.html)
   for a fuller explanation). Defaults to *default*.
 * `allow_existing` - Set to true if you want to allow the VM to use a
-  pre-existing disk.  This is useful for sharing disks between VMs, e.g. in
-  order to simulate shared SAN storage. Shared disks removed only manually. If
-  not exists - will created. If exists - using existed.
+  pre-existing disk. If the disk doesn't exist it will be created.
+  Disks with this option set to true need to be removed manually.
+* `shareable` - Set to true if you want to simulate shared SAN storage.
 
 The following example creates two additional disks.
 
@@ -606,6 +606,15 @@ Vagrant.configure("2") do |config|
 end
 ```
 
+For shared SAN storage to work the following example can be used:
+```ruby
+Vagrant.configure("2") do |config|
+  config.vm.provider :libvirt do |libvirt|
+    libvirt.storage :file, :size => '20G', :path => 'my_shared_disk.img', :allow_existing => true, :shareable => true, :type => 'raw'
+  end
+end
+```
+
 ### Reload behavior
 
 On `vagrant reload` the following additional disk attributes are updated in

lib/vagrant-libvirt/action/create_domain.rb

@@ -191,7 +191,8 @@ module VagrantPlugins
 
           @disks.each do |disk|
             msg = " -- Disk(#{disk[:device]}):     #{disk[:absolute_path]}"
-            msg += ' (shared. Remove only manually)' if disk[:allow_existing]
+            msg += ' Shared' if disk[:shareable]
+            msg += ' (Remove only manually)' if disk[:allow_existing]
             msg += ' Not created - using existed.' if disk[:preexisting]
             env[:ui].info(msg)
           end

lib/vagrant-libvirt/config.rb

@@ -376,6 +376,7 @@ module VagrantPlugins
           :bus => options[:bus],
           :cache => options[:cache] || 'default',
           :allow_existing => options[:allow_existing],
+          :shareable => options[:shareable],
         }
 
         @disks << disk  # append

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

@@ -75,6 +75,9 @@
       <driver name='qemu' type='<%= d[:type] %>' cache='<%= d[:cache] %>'/>
       <source file='<%= d[:absolute_path] %>'/>
       <target dev='<%= d[:device] %>' bus='<%= d[:bus] %>'/>
+      <% if d[:shareable] %>
+        <shareable/>
+      <% end %>
 <%# this will get auto generated by libvirt
       <address type='pci' domain='0x0000' bus='0x00' slot='???' function='0x0'/>
 -%>