Rather than returning a "char *" indicating perhaps some sized set of
characters that is NUL terminated, alter the function to return 0 or -1
for success/failure and add two parameters to handle returning the
buffer and it's size.

The function no longer encodes the returned secret, rather it returns
the unencoded secret forcing callers to make the necessary adjustments.

Alter the callers to handle the adjusted model.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

Call the internal driver callbacks rather than the public APIs to avoid
calling unnecessarily the error dispatching code and don't overwrite
the error messages provided by the APIs. They are good enough to
describe which secret is missing either by UUID or the usage (basically
name).

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