[VMXASSIST] Extensions to vmxassist to handle additional mode switch cases.

[VMXASSIST] Extensions to vmxassist to handle additional mode switch cases.

Extends vmxassist to handle two cases related to mode switching found
while experimenting with different boot loaders.

The first case is use of the JMP instruction with memory location
operands to complete a switch to protected mode. This patch adds
emulation for this form of the JMP instruction to vmxassist.

The second case is where boot loader code does not save/restore a
non-zero SS register across a protected mode traversal.
Zeroing the SS register in vmxassist results in all sorts of problems
in the domU after returning back to real mode. This patch stores
segment register values before entering protected mode and correctly
restores the old values (instead of an incorrect zero value) when
reentering real mode.

Signed-off-by: Kevin Tronkowski <ktronkowski@virtualiron.com>