Update:
This doc is better:
https://wiki.libvirt.org/page/VirtualNetworking
I must correct myself: I did not try pinging from other PCs in physical LAN to virtual guest, only from openSUSE host.

Still doc insist in the fact libvirt creates a "default" virtual network (in NAT mode) when installed and started. This was clearly not my case. Could it be because I accepted Yast's advise of creating bridges back when installing KVM stuff?

Yet, still in need of help for the other doubts:

Why still 2 virbr0 devices?
When configuring the bridge in virt-manager I left forward to "any physical device". Is it actually taking any *working* physical device that it detects, or what?
Is brctl really the only way to disable STP on bridges?