wic: do not overwrite autogenerated /etc/fstab with original too early

DirectImageCreator.__write_fstab() generates new /etc/fstab in sysroot
with rootfs contents. The fstab entries are generated base on the
initialn contents of /etc/fstab, plus any extra (other than / or
/boot) partitions listed in *.wks. A backup of original /etc/fstab is
done in a temp location. Subsequent call to __restore_fstab() restores
the backup copy, replacing the autogenerated one.

Calling __restore_fstab() before Wic_PartData.prepare() brings back the
original fstab before the partition image file actually is created. As
such, the autogenerated /etc/fstab will not make it to the partition.

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>