p2v: Add scripts to make disk or kickstart-based P2V ISO.

Two scripts are added, replacing the make targets from the
previous commit:

virt-p2v-make-disk: Builds a virt-p2v disk or USB key using
virt-builder.

virt-p2v-make-kickstart: Builds a kickstart file (only) which can be
fed to livecd-creator or similar tools on Red Hat-derived distros.

Kickstart generation is slightly different: Instead of requiring the
downstream 'libguestfs-pvhelper' package, the kickstart now embeds the
virt-p2v binary(!) making it self-contained and downstream packaging
simpler.

In addition there are documentation changes.

run.in

@@ -92,6 +92,12 @@ if [ -z "$XDG_CONFIG_DIRS" ]; then
     export XDG_CONFIG_DIRS
 fi
 
+# virt-p2v-make-* data directory.
+if [ -z "$VIRT_P2V_DATA_DIR" ]; then
+    VIRT_P2V_DATA_DIR="$b/p2v"
+    export VIRT_P2V_DATA_DIR
+fi
+
 # For Perl.
 if [ -z "$PERL5LIB" ]; then
     PERL5LIB="$b/perl/blib/lib:$b/perl/blib/arch"