v2v: rhv-upload-plugin: Fix name error

"http" and "transfer" variables were missing in emulate_zero, so the
code would fail with NameError.  This can happen only when communicating
with old imageio versions not supporting the "zero" feature.

Testing with qemu-img 2.12 show that we never send emulated zero request
because of the highestwrite mechanism, but it can break with older
qemu-img-rhev used on RHEL.

v2v/rhv-upload-plugin.py

@@ -338,6 +338,9 @@ def zero(h, count, offset, may_trim):
     r.read()
 
 def emulate_zero(h, count, offset):
+    http = h['http']
+    transfer = h['transfer']
+
     # qemu-img convert starts by trying to zero/trim the whole device.
     # Since we've just created a new disk it's safe to ignore these
     # requests as long as they are smaller than the highest write seen.