create_instance passed the loader list where find_uefi_path_for_arch
expects the machine type. The domain capabilities were then read for
the default machine (pc) instead of the chosen one: on a RHEL host a q35
VM was offered /usr/share/OVMF/OVMF_CODE.secboot.fd, which q35 does not
list, instead of /usr/share/edk2/ovmf/OVMF_CODE.fd. The machine type is
passed now.
The page also read dom_caps["loader_enums"] into an unused variable;
on a host without loader support the key is missing and the page failed
with a KeyError. The line is removed.
A unit test covers the call and the host without loader support (it
fails on the previous code). A live test creates a q35 VM with the
offered UEFI firmware, checks that libvirt lists that loader for q35 and
starts the VM; it fails on the previous code.
