Unify register representations across all Hypervisor implementations

[dblnz]

Currently, registers for the KVMDriver, HypervLinuxDriver and HypervWindowsDriver are handled in a similar manner.
It would be helpful to unify these registers in as type-safe a way as possible.

Some features of this unified API:

• Support for just the registers we need (e.g. RSP, RIP, pagetable-related registers, etc...)
• Use of GuestPtr for register values as appropriate (e.g. RSP which defines a stack pointer, which can only be a guest address)
• Prevents callers from setting any unsupported register values
• Ensures all required registers are set before a hypervisor can use them
• No support for optional registers. The registers are either all set or none are set

[danbugs]

Related to #168