selinux.restorecon: encode filename to utf-8 string

selinux python bindings take only a char * as input, not a unicode
string (C.f. https://bugzilla.redhat.com/908916). For this reason, we
need to encode a potential unicode string into a utf-8 char string.

Change-Id: Ibca42d47d5a241f864fa5818996734cce9955fe6
Bug-Url: https://bugzilla.redhat.com/1128074
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31388
Reviewed-by: Dan Kenigsberg <danken@redhat.com>