vsprintf: fix io/mem resource width

The leading "0x" consumes field width, so leave space for it in addition to
the 4 or 8 hex digits.  This means we'll print "0x0000-0x01df" rather than
"0x00-0x1df", for example.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>