virt-manager as well as virt-install creates a completely new VM which does not know about the UEFI configuration of my existing VM.
The problem seemed to be --boot uefi. I need
--boot loader=/.../OVMF_CODE.fd,loader.readonly=yes,loader.type=pflash,nvram.template=/.../OVMF_VARS.fd,loader_secure=n
But that leaves me to the next issue → Packer-build VM does not start: Impossible to rename file '(null).new' to '(null)': Bad address