[virtio] Allow for long delays in processing transmit queue submissions

When using QEMU with the KVM accelerator, MMIO writes to the queue
doorbell register are likely to hit an ioeventfd region.  KVM will
signal readiness on the ioeventfd file descriptor (which will
eventually wake up the QEMU userspace process to handle the MMIO
write) and then immediately resume execution of the iPXE guest.

This can result in high latencies in processing submitted descriptors.
With the small transmit queue fill level used by iPXE, this can easily
overrun the transmit queue and result in large numbers of dropped
transmissions.

Increase the transmit queue fill level to utilise the whole queue if
needed, and use the transmission deferral mechanism to avoid dropping
packets when high latencies occur during operation.

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