[xen] Support scatter-gather to allow for jumbo frames

[xen] Support scatter-gather to allow for jumbo frames

The use of jumbo frames for the Xen netfront virtual NIC requires the
use of scatter-gather ("feature-sg"), with the receive descriptor ring
becoming a list of page-sized buffers and the backend using as many
page buffers as required for each packet.

Since iPXE's abstraction of an I/O buffer does not include any sort of
scatter-gather list, this requires an extra allocation and copy on the
receive datapath for any packet that spans more than a single page.

This support is required in order to successfully boot an AWS EC2
virtual machine (with non-enhanced networking) via iSCSI if jumbo
frames are enabled, since the netback driver used in EC2 seems not to
allow "feature-sg" to be renegotiated once the Linux kernel driver
takes over.

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