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: H. Peter Anvin <hpa@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>