osdep.c: Fix accept4 fallback

Commit 3a03bfa5 added a fallback in case the Linux kernel running qemu is older
than the kernel of the build system. Unfortunately, v1 was committed instead of
v2, so the code has a bug that was revealed in the review (checking for the
wrong error code).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
