qemu: improve compile-time check of qemuBlockjobState mapping

qemu: improve compile-time check of qemuBlockjobState mapping

Asserting the value we set four lines earlier in qemuBlockjobState
doesn't buy us any safety (if the public header adds a value, we end
up skipping that value without the compiler warning us of our gap);
what we really want is to assert that the value auto-assigned by the
compiler matches the actual last value in the public headers (as was
done below for qemuBlockJobType).  Add useful comments while at it.

Signed-off-by: Eric Blake <eblake@redhat.com>
ACKed-by: Peter Krempa <pkrempa@redhat.com>