[roll] Roll fuchsia [kernel][x86][hypervisor] Restore host registers when vmlaunch fails.

We currently do not correctly restore the host registers when
a `vmlaunch` or `vmresume` command fails, but instead leave callee-saved
registers as their guest-register values.

This CL changes the structure of the `vmx_enter` code, allowing us to
use the same restore path for both the success and failure cases:

  * Instead of `vmx_enter` calling `vmx_exit`, we rename the existing
    `vmx_enter` to `vmx_enter_asm`, and make a new function `vmx_enter`
    call into both `vmx_enter_asm` and perform the existing `vmx_exit`
    functionality.

  * In `vmx_enter_asm`, instead of storing host callee-saved registers
    and return address in the vmx_state struct, we just save them on the
    stack like a normal function.

    This makes the `vmx_enter_asm` function look more like a normal
    function, and avoids some juggling of RIP in the entry/exit of the
    function.

  * Similarly, we save rflags directly on the host's stack, avoiding
    having to push/pop them in and out of the vmx_state struct.

  * The function's exit code is produced earlier and saved directly in
    RAX, allowing both the success and failure paths to share the host
    state restore path.

  * Finally, we combine the existing `vmx_enter`/`vmx_exit_entry`
    functions into just the one, to avoid having to jump from vmx_enter
    into the middle of vmx_exit_entry in the failure case.

    We still need a symbol in the middle of vmx_enter to write as the
    exit point in the VMCS, but from the debugger's perspective
    `vmx_enter_asm` becomes a single function.

Additionally, we add a test exercising the failure path in kernel unit
tests.

Original-Bug: 75942
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/526553
Original-Revision: feaf39c272a662b46385dedc50810d41d24a0775
GitOrigin-RevId: 29bfe34c0e74f5e45d74d14a35561da756536e71
Change-Id: If106ef0cf00e863603000579e7419cedec1dcb7e