Back in 2008 when this line of util.h was written, gnulib's verify
module didn't allow the use of multiple verify() in one file
in combination with our choice of gcc -W options.  But that has
since been fixed in gnulib, and newer gnulib even maps verify()
to the C1x feature of _Static_assert, which gives even nicer
diagnostics with a new enough compiler, so we might as well go
with the simpler verify().

* src/util/util.h (VIR_ENUM_IMPL): Use simpler verify, now that
gnulib module is smarter.