cgroup: only write one device entry at a time

We seem to have some reports of:

Error (cgroup.c:1193): cg: Failed writing c *:* m
b *:* m
c 1:3 rwm
c 1:5 rwm
c 1:7 rwm
c 5:0 rwm
c 5:2 rwm
c 1:8 rwm
c 1:9 rwm
c 136:* rwm
c 10:229 rwm to devices//lxc/c7/devices.allow: Argument list too long

so let's try and write these lines one by one, instead of simply advancing
the pointer along in the string. It's probably cleaner this way anyway :)

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Adrian Reber <adrian@lisas.de>
Tested-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>