Enable more mkosi options

Enable more mkosi options

- HostonlyInitrd reduces the size of the initramfs which speeds up
QEMU boot times.
- Autologin and deleting the root password remove
the manual login step whenever starting the systemd-nspawn container
or QEMU vm.
- NetworkVeth passes --network-veth to the nspawn container when
using mkosi boot and emulates --network-veth with a tap device
and networkd's 80-vm-vt.network builtin network file when using
mkosi qemu. --network-veth makes it easier to test network and
resolved in mkosi's nspawn container or QEMU vm.