slirp: Enforce host-side user of smb share

Windows 7 (and possibly other versions) cannot connect to the samba
share if the exported host directory is not world-readable. This can be
resolved by forcing the username used for access checks to the one
under which QEMU and smbd are running.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
