Don't fail chmod/chdir if a file doesn't exist

We don't use -f since we want to detect other errors (ro filesystem,
etc.)

Closes: #726203

debian/libvirt-bin.postinst

@@ -67,26 +67,26 @@ add_statoverrides()
 
     for dir in ${ROOT_DIRS}; do
         if ! dpkg-statoverride --list "${dir}" >/dev/null 2>&1; then
-            chown root:root "${dir}"
-            chmod 0711 "${dir}"
+            [ ! -e "${dir}" ] || chown root:root "${dir}"
+            [ ! -e "${dir}" ] || chmod 0711 "${dir}"
         fi
     done
 
     for dir in ${QEMU_DIRS}; do
         if ! dpkg-statoverride --list "${dir}" >/dev/null 2>&1; then
-            chown libvirt-qemu:libvirt-qemu "${dir}"
-            chmod 0750 "${dir}"
+            [ ! -e "${dir}" ] || chown libvirt-qemu:libvirt-qemu "${dir}"
+            [ ! -e "${dir}" ] || chmod 0750 "${dir}"
         fi
     done
 
     if ! dpkg-statoverride --list "${SANLOCK_DIR}" >/dev/null 2>&1; then
-        chown root:root "${SANLOCK_DIR}"
-        chmod 0700 "${SANLOCK_DIR}"
+        [ ! -e "${dir}" ] || chown root:root "${SANLOCK_DIR}"
+        [ ! -e "${dir}" ] || chmod 0700 "${SANLOCK_DIR}"
     fi
 
     if ! dpkg-statoverride --list "${QEMU_CONF}" >/dev/null 2>&1; then
-        chown root:root "${QEMU_CONF}"
-        chmod 0600 "${QEMU_CONF}"
+        [ ! -e "${dir}" ] || chown root:root "${QEMU_CONF}"
+        [ ! -e "${dir}" ] || chmod 0600 "${QEMU_CONF}"
     fi
 }