newvm: Allow setting cfgpol branch

The script that runs on first boot of a new machine that triggers
host provisioning can read the name of the configuration policy branch
to checkout from the QEMU firmware configuration option.  This commit
adds a `--cfg-branch` argument to `newvm.sh` that sets that value.  This
will be useful for testing new policy on a new VM.

newvm.sh

@@ -75,6 +75,13 @@ while [ $# -gt 0 ]; do
     --domain=*)
         dnsdomain=${1#=*}
         ;;
+    --cfg-branch)
+        shift
+        cfg_branch=$1
+        ;;
+    --cfg-branch=*)
+        cfg_branch=${1#*=}
+        ;;
     --group)
         shift
         groups_xml="${groups_xml}<group name=\"${1}\"/>"
@@ -163,6 +170,10 @@ set -- \
     --boot uefi \
     "$@"
 
+if [ -n "${cfg_branch}" ]; then
+    set -- "$@" --qemu-commandline="-fw_cfg opt/dch/cfg-branch,string=${cfg_branch}"
+fi
+
 if $console; then
     set -- "$@" --autoconsole text
 else