public/hvm_xs_strings.h: Fix ABI regression for OEM SMBios strings

public/hvm_xs_strings.h: Fix ABI regression for OEM SMBios strings

The old code for OEM SMBios strings was:

        char path[20] = "bios-strings/oem-XX";
        path[(sizeof path) - 3] = '0' + ((i < 10) ? i : i / 10);
        path[(sizeof path) - 2] = (i < 10) ? '\0' : '0' + (i % 10);

Where oem-1 thru 9 specifically had no leading 0.

However, the definition of HVM_XS_OEM_STRINGS specifically requires leading
0s.

This regression was introduced by the combination of c/s 4d23036e709627 and
e64c3f71ceb662

I realise that this patch causes a change to the public headers.  However I
feel it is justified as:

* All toolstacks used to have to embed the magic string (and almost certainly
  still do)
* If by some miriacle a new toolstack has started using the new define will
  continue to work.
* The only intree consumer of the define is hvmloader itself.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>