meson: fix missing preprocessor symbols

meson: fix missing preprocessor symbols

While most libraries do not need a CONFIG_* symbol because the
"when:" clauses are enough, some do.  Add them back or stop
using them if possible.

In the case of libpmem, the statement to add the CONFIG_* symbol
was still in configure, but could not be triggered because it
checked for "no" instead of "disabled" (and it would be wrong anyway
since the test for the library has not been done yet).

Reported-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Fixes: 587d59d6cc ("configure, meson: convert virgl detection to meson", 2021-07-06)
Fixes: 83ef16821a ("configure, meson: convert libdaxctl detection to meson", 2021-07-06)
Fixes: e36e8c70f6 ("configure, meson: convert libpmem detection to meson", 2021-07-06)
Fixes: 53c22b68e3 ("configure, meson: convert liburing detection to meson", 2021-07-06)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>