Simplify instruction restart logic in bhyve.

Keep track of the next instruction to be executed by the vcpu as 'nextrip'.
As a result the VM_RUN ioctl no longer takes the %rip where a vcpu should
start execution.

Also, instruction restart happens implicitly via 'vm_inject_exception()' or
explicitly via 'vm_restart_instruction()'. The APIs behave identically in
both kernel and userspace contexts. The main beneficiary is the instruction
emulation code that executes in both contexts.

bhyve(8) VM exit handlers now treat 'vmexit->rip' and 'vmexit->inst_length'
as readonly:

• Restarting an instruction is now done by calling 'vm_restart_instruction()' as opposed to setting 'vmexit->inst_length' to 0 (e.g. emulate_inout())
• Resuming vcpu at an arbitrary %rip is now done by setting VM_REG_GUEST_RIP as opposed to changing 'vmexit->rip' (e.g. vmexit_task_switch())

Differential Revision: https://reviews.freebsd.org/D1526
Reviewed by: grehan
MFC after: 2 weeks