[bootloader] Fix missing quotes in grub configuration for Xen

When trying to pass several Xen parameters in hypervisor_params (to fix
bug 11436), the postinstall fails:

    /usr/sbin/grub-mkconfig: 36: /etc/default/grub: no-bootscrub: not found

Adding the missing quotes fixes it.

lib/g5k-postinstall/bootloader.rb

@@ -72,7 +72,7 @@ def install_grub2
       f.write("#{grub_default}\n")
       f.write("GRUB_DISABLE_OS_PROBER=true\n")
       f.write("GRUB_DISABLE_LINUX_UUID=true\n")
-      f.write("GRUB_CMDLINE_XEN=#{hypervisor_params}\n") if os_kind == 'xen'
+      f.write("GRUB_CMDLINE_XEN=\"#{hypervisor_params}\"\n") if os_kind == 'xen'
       f.write("XEN_OVERRIDE_GRUB_DEFAULT=1\n") if os_kind == 'xen'
     end