x86, doc: Add a bootloader ID for OVMF

OVMF (an implementation of UEFI based on TianoCore used in virtual
environments) now has the ability to boot Linux natively; this is used
for "qemu -kernel" and similar things in a UEFI environment.

Accordingly, assign it a bootloader ID.
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>

Documentation/x86/boot.txt

@@ -390,6 +390,7 @@ Protocol:	2.00+
 	F  Special		(0xFF = undefined)
        10  Reserved
        11  Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
+       12  OVMF UEFI virtualization stack
 
   Please contact <hpa@zytor.com> if you need a bootloader ID
   value assigned.