Simplify the meaning of return values from the bhyve's inout handlers.

[neel]

Summary

inout handlers now return 0 on success and non-zero on failure. This also
helps to eliminate VMEXIT_POWEROFF and VMEXIT_RESET as return values from
VM-exit handlers.

Test Plan

Verified no regression in atkbd and 0xcf9 reset.
Verified no regression in ACPI poweroff.
Verified no regression in #GP, #PF and #AC generation by inout string emulation.

[jhb]

Looks good to me.

[grehan]

Looks fine.