This reverts commit f38d553e.

Gnulib's make coverage (or init-coverage, build-coverage, gen-coverage)
is not a 1-1 replacement for the original configure option. Our old
--enable-test-coverage seems to be close to gnulib's make build-coverage
except gnulib runs lcov in that phase and the build actually fails for
me even before lcov is run. And since we want to be able to just build
libvirt without running lcov, I suggest reverting to our own
implementation.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Acked-By: NEric Blake <eblake@redhat.com>