[librm] Speed up protected-mode calls under KVM

When making a call from real mode to protected mode, we save and
restore the global and interrupt descriptor table registers.  The
restore currently takes place after returning to real mode, which
generates two EXCEPTION_NMIs and corresponding VM exits when running
under KVM on an Intel CPU.

Avoid the VM exits by restoring the descriptor table registers inside
prot_to_real, while still running in protected mode.

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