[librm] Allow interrupts in protected mode

When running in a virtual machine, switching to real mode may be
expensive.  Allow interrupts to be enabled while in protected mode and
reflected down to the real-mode interrupt handlers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[comboot] Use built-in interrupt reflector

We now have the ability to handle interrupts while in protected mode,
and so no longer need to set up a dedicated interrupt descriptor table
while running COM32 executables.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[pcbios] Do not switch to real mode to check for timer interrupt

The currticks() function is called at least once per TCP packet, and
so is performance-critical.  Switching to real mode just to allow the
timer interrupt to fire is expensive when running inside a virtual
machine, and imposes a significant performance cost.

Fix by enabling interrupts without switching to real mode.  This
results in an approximately 100% increase in download speed when
running under KVM.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[pcbios] Do not switch to real mode to sleep the CPU

Now that we can handle interrupts while in protected mode, there is no
need to switch to real mode just to halt the CPU.

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