With qemu, use -nic user option, and set up networking as follows

ip address add 10.0.2.15/24 dev eth0
ip link set dev eth0 up
ip route add 10.0.2.0/24 dev eth0
ip route add default via 10.0.2.2 dev eth0

Also, your /etc/resolv.conf should contain

nameserver 10.0.2.3

From there, you should be able to apt install dhcpcd-base or whatever package(s) you want.

With virtualbox, you could change the network to bridged, and set up static networking as though it were a physical device on your LAN.  For example, if your LAN uses 192.168.1.0/24 with a gateway of 192.168.1.1 (I think this is a common default setup), you could do something like this

ip address add 192.168.1.20/24 dev eth0
ip link set dev eth0 up
ip route add 192.168.1.0/24 dev eth0
ip route add default via 192.168.1.1 dev eth0

Obviously, you'd want to use an address not in use by another device.  That was just an example.

# guestmount -a devuan.qcow2 -i /mnt

Now you can copy dhcpcd_*.deb and its dependencies to /mnt/root or wherever you want within /mnt, and safely remove the disk afterwards:

# guestunmount /mnt

Boot the image and install the packages via:

# dpkg -i /root/*.deb

You can also configure /etc/network/interfaces if you wish:

auto eth0
allow-hotplug eth0
iface eth0 inet dhcp

https://dev1galaxy.org/viewtopic.php?id=6283

But... I cannot get inside of the qcow2 with an installation nor can I install anything remotely needed to get it working.

So I wish to ask,

Is there a way to forcibly give internet to a vm? In either virt-manager, qemu, or virtualbox.

???