bhyve: Preserve upper RAX bits for byte and word input

[hayzam_gmail.com]

Summary

Scalar IN merges the input into a 32-bit EAX temporary and writes that
value to RAX. This clears bits 63:32 for byte and word inputs, which must
preserve the bits outside AL or AX.

Use vie_update_register() to update the accumulator at the operand width.
This preserves the remaining bits for byte and word inputs and retains
zero-extension for dword inputs.

Sponsored by: Alchemilla Ventures Private Limited

[markj]

Does emulate_inout_port() in the kernel need the same change?

[hayzam_gmail.com]

Fix RAX preservation in the kernel scalar IN handler too.

[hayzam_gmail.com]

In D61498#1409742, @markj wrote:

Does emulate_inout_port() in the kernel need the same change?

Yes, thanks for catching that. Updated emulate_inout_port() to use vie_update_register() too.