I'm not sure what versions of qemu lack virtio-serial support, but
it's not ones that we have ever seen, so remove this test. Also
virtio-serial doesn't work without kernel support, but we never tested
that.
If qemu or the guest kernel really lacks virtio-serial (likely it has
been compiled out) then we'll see it as soon as someone uses
libguestfs.
This also removes QEMU_OPTIONS_FOR_CONFIGURE, and the test of the qemu
version, since both are no longer used after we remove this test.
Run this command across the source:
perl -pi.bak -e 's/(20[012][0-9])-20[12][012]/$1-2023/g' `git ls-files`
and remove changes to po{,-docs}/*.po{,t} (these will be regenerated
later when we run 'make dist').
