udev: allow normal user to access /dev/xen/xenbus for vchan connections

Recently we've switched all xenstore access to the new interface
(instead of deprecated /proc/xen/xenbus). Mostly because of deadlock in
/proc/xen/xenbus implementation.

udev/udev-qubes-misc.rules

@@ -3,3 +3,4 @@ KERNEL=="xen/evtchn", MODE="0660", GROUP="qubes"
 KERNEL=="xen/gntdev", MODE="0660", GROUP="qubes"
 KERNEL=="xen/gntalloc", MODE="0660", GROUP="qubes"
 KERNEL=="xen/privcmd", MODE="0660", GROUP="qubes"
+KERNEL=="xen/xenbus", MODE="0660", GROUP="qubes"