Refactor: fencer: Best practices in update_cib_stonith_devices()

• Remove some nesting.
• Use const where possible.
• Improve variable names and scope.
• Use a macro for repeated string literal.
• Use sizeof() - 1 instead of strlen() for string literal.
• Remove unnecessary escaping of single quote within double quotes.
• Copy only what is needed of the xpath string, if anything.
• Log assertion on malformed patchset, since we created it in cib_perform_op().
• Add a comment to clarify why we're (apparently) doing all this in the first place.

Keep the watchdog_device_update() comment as-is because I haven't looked
into it.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>