[RFC] proposed 'lem' patch to improve behaviour under emulation

This patch implements two features for the 'lem' driver that
greatly improve the throughput under proper hypervisor.
This is joint work with Vincenzo Maffione and Giuseppe Lettieri,
I am posting it here for review, will then commit it 
if there are no objections.
 
The first change is to implement a sysctl to access the 'itr'
interrupt moderation register for the devices supported by this
driver. It is little more than adding a struct into the device
descriptor, and one line to create the dynamic sysctl entry, same
as it is done for the other mitigation registers.
 
The second change is more interesting and has huge benefits on througput.

Under virtualization, "VM exits" (which happen every time there is
an access to a register of the emulated peripheral) are extremely
expensive.  In the tx path of the 'lem' driver, there is a write
to the TDT register on every packet sent. 

The patch we propose, if enabled through a sysctl (defaults off, 
so no change from current behaviour) defers writes to the TDT 
register when there is a pending transmit interrupt. 
This means that, together with proper emulation of interrupt
mitigation on the hypervisor side, the number of VM exits
is dramatically reduced. To give you an idea, on a modern
system with qemu-kvm and companion patches, UDP throughput is
 
					KVM		QEMU
standard KVM, standard driver            20 Kpps	 6.3 Kpps
modified KVM, standard driver            37 Kpps	28 Kpps
modified KVM, modified driver           200 Kpps	34 Kpps
 
As you can see, on kvm this change gives a 5x speedup to the tx path,
which combines nicely with the 2x speedup that comes from supporting
interrupt mitigation alone in the hypervisor. Without kvm (or kqemu ?)
the benefits are much lower, as the guest becomes too slow.

Patch follows. It would be good if people with real hardware
using the 'lem' driver could test it to make sure it does no
harm on their devices (in any case the sysctl variable
dev.em.0.mit_enable must be set to explicitly enable it
at runtime).

(for those curious to test it under kvm, i am also attaching a
patch that you need to apply to qemu in order to exploit the
effect of interrupt mitigation; it is a followup of a similar
patch i posted in july to the qemu mailing list, and i will
post it the update there as well, shortly. Unfortunately
we do not have kvm on freebsd..)

cheers
luigi