netback: remove dead code

Remove the xen_net_read_mac function since it's not used anymore.

Sponsored by: Citrix Systems R&D
vt(4): The offset to center the text area is per-window now

The previous global offset, based on the last loaded font, had no
meaning for other windows. This caused a shifted text area, often partly
out-of-screen.

MFC after:	1 week
xen: fix incorrectly accounted free

Fix some frees incorrectly assigned to M_XENBUS when the memory is
allocated with M_XENSTORE.

Sponsored by: Citrix Systems R&D
MFC after:	1 week

dev/xen/blkback/blkback.c:
 - Fix incorrect frees.