vmm(4): send all port io emulation to userland

vmm(4): send all port io emulation to userland

Simplify things by sending any io exits from IN/OUT instructions
to userland instead of trying to emulate anything in the kernel.
vmm was sending most pertinent exits to vmd anyways, so this
functionally changes little.

An added benefit is this solves an issue reported by tb@ where i386
OpenBSD guests would probe for a pc keyboard repeatedly and cause
excessive vm exits. (The emulation in vmm was not properly handling
these port reads.)

While here, make the assignment of the VEI_DIR_{IN,OUT} enum values
not assume the underlying integer the compiler may assign.

ok mlarkin@