nbd: Fix nbd_establish_connection()'s return value

nbd: Fix nbd_establish_connection()'s return value

unix_connect_opts() and inet_connect_opts() do not necessarily set errno
(if at all); therefore, nbd_establish_connection() should not literally
return -errno on error.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <1424887718-10800-4-git-send-email-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>