vapic: fix vapic option rom sizing

An option rom is required to be aligned to 512 bytes, and to have some space for a signature.

---
 pc-bios/optionrom/vapic.S |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

index 3c8dcf1..97fa19c 100644
@@ -323,4 +323,8 @@ up_set_tpr_poll_irq:
 
 vapic:
 . = . + vapic_size
+
+.byte 0  # reserve space for signature
+.align 512, 0
+
 _end:

1.7.3.1