block/ssh: Avoid segfault if inet_connect doesn't set errno.

On some (but not all) systems:

  $ qemu-img create -f qcow2 overlay -b ssh://xen/
  Segmentation fault

It turns out this happens when inet_connect returns -1 in the
following code, but errno == 0.

  s->sock = inet_connect(s->hostport, errp);
  if (s->sock < 0) {
      ret = -errno;
      goto err;
  }

In the test case above, no host called "xen" exists, so getaddrinfo fails.

On Fedora 22, getaddrinfo happens to set errno = ENOENT (although it
is *not* documented to do that), so it doesn't segfault.

On RHEL 7, errno is not set by the failing getaddrinfo, so ret =
-errno = 0, so the caller doesn't know there was an error and
continues with a half-initialized BDRVSSHState struct, and everything
goes south from there, eventually resulting in a segfault.

Fix this by setting ret to -EIO (same as block/nbd.c and
block/sheepdog.c).  The real error is saved in the Error** errp
struct, so it is printed correctly:

  $ ./qemu-img create -f qcow2 overlay -b ssh://xen/
  qemu-img: overlay: address resolution failed for xen:22: No address associated with hostname

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Reported-by: Jun Li
BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1147343
Signed-off-by: Jeff Cody <jcody@redhat.com>
