HACKING already mentions that comparisons against literal 'true'
are unsafe; for consistency, also shorten comparisons against 'false'.

* src/conf/domain_conf.c (virDomainNetDefParseXML): Simplify.
* src/nwfilter/nwfilter_gentech_driver.c
(virNWFilterDomainFWUpdateCB): Likewise.
* tools/virsh.c (cmdVolDownload, vshCommandRun, vshPrintExtra):
Likewise.