Eliminate large stack buffer in doTunnelSendAll

Eliminate large stack buffer in doTunnelSendAll

doTunnelSendAll function (used by QEMU migration) uses a 64k buffer on
the stack, which could be problematic. This patch replaces that with a
buffer from the heap.

While in the neighborhood, this patch also improves error reporting in
the case that saferead fails - previously, virStreamAbort() was called
(resetting errno) before reporting the error. It's been changed to
report the error first.

* src/qemu/qemu_driver.c: fix doTunnelSendAll() to use a malloc'ed
  buffer