Adjust permissions of /dev/xen/hypercall

Starting with Linux 4.18, there is new device node for issuing
hypercalls from user space - /dev/xen/hypercall (partially duplicating
functionality of /dev/xen/privcmd). New Xen tools (4.12) make use of it,
so make it available for them. Otherwise such tools will fail the
operation (there is no fallback to privcmd on EACCESS).

udev/udev-qubes-misc.rules

@@ -4,3 +4,4 @@ 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"
+KERNEL=="xen/hypercall", MODE="0660", GROUP="qubes"