Always include kernfs on Xen.

NetBSD/xen provides some APIs via kernfs, and kernfs being always
present on rumprun/xen allows us to do the same.

app-tools/rumpbake.conf

@@ -21,6 +21,7 @@ LIBS_VND="rumpdev_vnd"
 LIBS_RND="rumpdev_rnd"
 
 LIBS_FS="rumpfs_ffs rumpfs_cd9660 rumpfs_tmpfs"
+LIBS_KERNFS="rumpfs_kernfs"
 LIBS_DISK="rumpdev_disk rumpdev"
 LIBS_NETINET="rumpnet_netinet6 rumpnet_netinet rumpnet_net rumpnet"
 LIBS_NETCONFIG="rumpnet_config"
@@ -32,12 +33,12 @@ LIBS_XEN="rumpxen_xendev rumpxen_tc"
 
 # Paravirtualized Xen
 addconfig xen_pv rumprun-xen \
-    RND FS DISK NETINET NETUNIX NETBPF NETCONFIG XEN XEN_NET
+    RND FS KERNFS DISK NETINET NETUNIX NETBPF NETCONFIG XEN XEN_NET
 
 # Paravirtualized Xen with PCI drivers
 # (not a whole lot of them, but some anyway)
 addconfig xen_pci rumprun-xen \
-    PCI PCI_NET RND FS DISK NETINET NETUNIX NETBPF NETCONFIG XEN XEN_NET
+    PCI PCI_NET RND FS KERNFS DISK NETINET NETUNIX NETBPF NETCONFIG XEN XEN_NET
 
 # Generic (PC-compatible) bare metal
 addconfig hw_generic rumprun-bmk \