Commit id 'fb2bd208' essentially copied the qemuGetSecretString
creating an libxlGetSecretString.  Rather than have multiple copies
of the same code, create src/secret/secret_util.{c,h} files and
place the common function in there.

Modify the the build in order to build the module as a library
which is then pulled in by both the qemu and libxl drivers for
usage from both qemu_command.c and libxl_conf.c