build: Use AC_PROG_AWK to define the right awk to call.

We actually call plain 'awk' in many different scripts, but this
commit does not change all of those.
configure: Move --with-default-backend test later in configure.ac.

For some reason this test was right at the top of the script.
p2v-iso: unset CDPATH, set -e at the top of the virt-builder-target script.
p2v-iso: Add a kickstart target.

This also changes the old p2v-iso/README file into a POD file
which is installed as a manual page.
Update BUGS.