Support setting mtu on public interfaces (#1700)

Setting mtu on management and private interfaces is supported. It should
also be supported on public interfaces.

docs/configuration.markdown

@@ -549,6 +549,9 @@ virtual network.
   help attach VLAN tags to specific VMs by adjusting the pattern to match. Note
   that you cannot use names reserved for libvirt's usage based on [documentation](
   https://libvirt.org/formatdomain.html#overriding-the-target-element).
+* `:libvirt__mtu` - MTU size for the Libvirt interface, if not defined, the
+  created network will use the Libvirt default (1500). VMs still need to configure
+  their internal interface MTUs.
 
 Additionally for public networks, to facilitate validating if the device provided
 can be used, vagrant-libvirt will check both the host interfaces visible to libvirt

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

@@ -23,6 +23,9 @@
   <%- elsif @driver_iommu %>
     <driver iommu='on' />
   <%- end %>
+  <% if @mtu %>
+  <mtu size="<%= @mtu %>" />
+  <% end %>
   <%- if @ovs %>
   <virtualport type='openvswitch'>
     <%- if @ovs_interfaceid %>