cURL 7.16.0 was released in October 2006.  Just remove code that is
in all likelihood not being used anywhere, and require the oldest version
found in currently supported distros, which is 7.29.0 from CentOS 7.

pkg-config is enough for QEMU, since it does not need extra information
such as the path for certicate authorities.  All supported platforms
today will all have pkg-config for curl, so we can drop curl-config.

Suggested-by: Daniel Berrangé <berrange@redhat.com>
Reviewed-by: Daniel Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>