Is there something like "VirtualBox Guest Additions" for QEMU/KVM?

Yes, the SPICE guest additions

https://www.spice-space.org/download.html#guest

Yes, there is something like guest additions in virt-manager - it just has to be done a little bit differently. To make it easy for you to see how it works, I'll provide you with some screenshots.

First change the ACL attributes of the libvirt-qemu user to give write permissions on the shared folder : sudo setfacl -R -m u:libvirt-qemu:rwx /<path-of-the-shared folder>
and the Vdisk : sudo setfacl -R -m u:libvirt-qemu:rwx /<path-of-the-virtual-disk>

In the example below I gave permissions for all separate mounted partitions to be flexible :
sudo setfacl -R -m u:libvirt-qemu:rwx /media/cl (as I said - an example of my setup)

Open the virtual machine in virt-manager - select the disk - mark Shareable - click on Apply.

Click on Add Hardware - select Filesystem - change the Mode to Mapped - click on Browse.

Click on Browse Local.

Select the folder you want to share with the guest operating system - click on Open.

Type /host into the field Target path - click on Finish.

Now boot the guest system, create a folder with the same name as the one you selected to be shared in the host system (in my example : share) in the /home directory of the guest system. Open a terminal and execute the following command to mount the shared folder in the guest :

sudo mount -t 9p -o trans=virtio,version=9p2000.L /host /home/<your-user-name>/<your-shared-folder-name>  

Et voilà - now you are having equivalent functionality as with the guest additions in VirtualBox.