qemu/slirp: Fix SMB security configuration on newer samba versions

The smb.conf automatically generated by qemu's -smb option fails on current
samba, because smbd rejects the security=share option with the following warning:

>   WARNING: Ignoring invalid value 'share' for parameter 'security'

Which makes it fall back to security=user without guest login.
This results in being unable to login to the samba server from the guest OS.

This fixes it by selecting 'user' explicitly and mapping
unknown users to guest logins.

Signed-off-by: Michael Buesch <m@bues.ch>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
