[libvirt] Miscellaneous fixes to build with -Werror

HACKING suggests compiling with --enable-compile-warnings=error before submitting any patches; however, current master fails for me on this account (CentOS 5.3; gcc 4.1.2). Please see attached. I suspect most of these should be uncontroversial -- but wonder if perhaps virStrcpy uses would be better converted to virStrcpyStatic rather than adding virStrcpy to the symbol list as done here, and am curious about whether the need for explicit initialization to silence a warning regarding qemudSetLogging's log_level indicates a bug in the macro later used to assign that value.