maint: avoid remaining sprintf uses

* cfg.mk (sc_prohibit_sprintf): New rule.
(sc_prohibit_asprintf): Avoid false positives.
* docs/hacking.html.in (Printf-style functions): Document the
policy.
* HACKING: Regenerate.
* .x-sc_prohibit_sprintf: New exemptions.
* Makefile.am (syntax_check_exceptions): Ship new file.
* src/vbox/vbox_tmpl.c (vboxStartMachine, vboxAttachUSB): Use
virAsprintf instead.
* src/uml/uml_driver.c (umlOpenMonitor): Use snprintf instead.
* tools/virsh.c (cmdDetachInterface): Likewise.
* src/security/security_selinux.c (SELinuxGenSecurityLabel):
Likewise.
* src/openvz/openvz_driver.c (openvzDomainDefineCmd): Likewise,
and ensure large enough buffer.

HACKING

@@ -538,6 +538,12 @@ virAsprintf, in util.h:
 This makes it so gcc's -Wformat and -Wformat-security options can do their
 jobs and cross-check format strings with the number and types of arguments.
 
+When printing to a string, consider using virBuffer for incremental
+allocations, virAsprintf for a one-shot allocation, and snprintf for
+fixed-width buffers. Do not use sprintf, even if you can prove the buffer
+won't overflow, since gnulib does not provide the same portability guarantees
+for sprintf as it does for snprintf.
+
 
 Use of goto
 ===========