slirp/smb: Replace constant strings by glib string

gcc 7 (on fedora 26) objects to many of the snprintf's
in the smb path and command creation because it can't
figure out that the smb_dir (i.e. the /tmp dir for the configuration)
is known to be short.

Replace all these fixed length buffers by g_str* functions that dynamically
allocate and use g_dir_make_tmp to make the directory.
(It's fairly new glib but we have a compat function for it).

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
