[intelxl] Reuse admin command descriptor and buffer for VF responses

[intelxl] Reuse admin command descriptor and buffer for VF responses

Remove the large static admin data buffer structure embedded within
struct intelxl_nic, and instead copy the response received via the
"send to VF" admin queue event to the (already consumed and completed)
admin command descriptor and data buffer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>