Enforce function declaration format in bash8

Check that function calls look like ^function foo {$ in bash8, and fix
all existing failures of that check.  Add a note to HACKING.rst

Change-Id: Ic19eecb39e0b20273d1bcd551a42fe400d54e938
diff --git a/tools/xen/build_xva.sh b/tools/xen/build_xva.sh
index fbbfd6f..cc3cbe1 100755
--- a/tools/xen/build_xva.sh
+++ b/tools/xen/build_xva.sh
@@ -42,7 +42,7 @@
 #
 GUEST_NAME="$1"
 
-function _print_interface_config() {
+function _print_interface_config {
     local device_nr
     local ip_address
     local netmask
@@ -68,7 +68,7 @@
     echo "  post-up ethtool -K $device tx off"
 }
 
-function print_interfaces_config() {
+function print_interfaces_config {
     echo "auto lo"
     echo "iface lo inet loopback"