Make it possible to set the path to the device model emulator.

On some systems (e.g. Debian) omitting this setting results in the
following error:

    Error while creating domain: Error saving the server: Call to
    virDomainDefineXML failed: invalid argument: could not find
    capabilities for domaintype=kvm

Which has been reported as:

    https://github.com/pradels/vagrant-libvirt/issues/539

lib/vagrant-libvirt/config.rb

@@ -71,6 +71,7 @@ module VagrantPlugins
       attr_accessor :kernel
       attr_accessor :cmd_line
       attr_accessor :initrd
+      attr_accessor :emulator_path
       attr_accessor :graphics_type
       attr_accessor :graphics_autoport
       attr_accessor :graphics_port
@@ -144,6 +145,7 @@ module VagrantPlugins
         @kernel            = UNSET_VALUE
         @initrd            = UNSET_VALUE
         @cmd_line          = UNSET_VALUE
+        @emulator_path     = UNSET_VALUE
         @graphics_type     = UNSET_VALUE
         @graphics_autoport = UNSET_VALUE
         @graphics_port     = UNSET_VALUE

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

@@ -55,6 +55,9 @@
   </features>
   <clock offset='utc'/>
   <devices>
+    <% if @emulator_path %>
+    <emulator><%= @emulator_path %></emulator>
+    <% end %>
     <% if @domain_volume_path %>
     <disk type='file' device='disk'>
       <driver name='qemu' type='qcow2' cache='<%= @domain_volume_cache %>'/>