buildserver: only set disk_bus/nic_model_type if set in config file

This makes vagrant/KVM just use the default if these are not set in the
makebuildserver.config.py.

fdroid/fdroidserver!585

buildserver/Vagrantfile

@@ -34,8 +34,12 @@ Vagrant.configure("2") do |config|
       libvirt.uri = "qemu:///system"
       libvirt.cpus = configfile["cpus"]
       libvirt.memory = configfile["memory"]
-      libvirt.disk_bus = configfile["libvirt_disk_bus"]
-      libvirt.nic_model_type = configfile["libvirt_nic_model_type"]
+      if configfile.has_key? "libvirt_disk_bus"
+        libvirt.disk_bus = configfile["libvirt_disk_bus"]
+      end
+      if configfile.has_key? "libvirt_nic_model_type"
+        libvirt.nic_model_type = configfile["libvirt_nic_model_type"]
+      end
     end
     config.vm.synced_folder './', '/vagrant', type: '9p'
     synced_folder_type = '9p'

makebuildserver

@@ -65,8 +65,6 @@ config = {
     'memory': 2048,
     'hwvirtex': 'off',
     'vm_provider': 'virtualbox',
-    'libvirt_disk_bus': 'virtio',
-    'libvirt_nic_model_type': 'virtio',
 }
 
 if os.path.isfile('/usr/bin/systemd-detect-virt'):