block/curl: Don't lose original error when a connection fails.

block/curl: Don't lose original error when a connection fails.

Currently if qemu is connected to a curl source (eg. web server), and
the web server fails / times out / dies, you always see a bogus EIO
"Input/output error".

For example, choose a large file located on any local webserver which
you control:

  $ qemu-img convert -p http://example.com/large.iso /tmp/test

Once it starts copying the file, stop the webserver and you will see
qemu-img fail with:

  qemu-img: error while reading sector 61440: Input/output error

This patch does two things: Firstly print the actual error from curl
so it doesn't get lost.  Secondly, change EIO to EPROTO.  EPROTO is a
POSIX.1 compatible errno which more accurately reflects that there was
a protocol error, rather than some kind of hardware failure.

After this patch is applied, the error changes to:

  $ qemu-img convert -p http://example.com/large.iso /tmp/test
  qemu-img: curl: transfer closed with 469989 bytes remaining to read
  qemu-img: error while reading sector 16384: Protocol error

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>