I want to make sure these VMs should be able to access the internet (through the host) but they should not be able to access the host or any other device on the LAN.
The first and simplest solution comes into my mind is to use the direct type interface (aka. macvtap).
<interface type='direct'>
<source dev='eth0' mode='private'/>
</interface> Note: "eth0" is the active interface on the host, with internet connection.
With macvtap, your VM will be in the same subnet as the host. If there is a dhcp server in this subnet, the VM will get a dhcp address.
With mode as "private", it can not access the endpoints (other VMs with the same setting: direct + eth0 + private).
The VM will have the same internet connectivity as its host, but the VM and host can not access each other.
But other devices on the same LAN can access the VM and vice versa (like other hosts in the same subnet as the host). Maybe you can use firewalld rules to fine tune it.
And I wonder if we block the connection from the VM to all the hosts in the same subnet, how can it access the internet as it can not access the default gateway?
Another solution is to use virtual network with forward mode as "open". With this mode, libvirt will not add any rules, so users can customize how it works by their needs, this may be more complicated.
Hope it helps.