Replace hardcoded padding size value for struct kvm_sync_regs
with #define SYNC_REGS_SIZE_BYTES.

Also update the value specified in api.txt from outdated hardcoded
value to SYNC_REGS_SIZE_BYTES.
Signed-off-by: NKen Hofsass <hofsass@google.com>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>