conf: set postParseFailed even without ALLOW_POST_PARSE_FAIL

conf: set postParseFailed even without ALLOW_POST_PARSE_FAIL

We allow the postParse callbacks to fail for some reasons (missing
emulator binary) when parsing the configs from /etc/libvirt.
In that case, def->postParseFailed is set to true and the post
parse callbacks are re-executed on domain startup.

However this bool was only set when virDomainDefPostParse was called
with the ALLOW_POST_PARSE_FAIL flag set. If the callback failed
again on domain startup, the bool would be reset and subsequent
startups would not attempt to reexecute the callback.

Signed-off-by: Ján Tomko <jtomko@redhat.com>