vmd(8): simplify vcpu logic, removing uart & vionet reads

vmd(8): simplify vcpu logic, removing uart & vionet reads

Remove legacy state handling on the ns8250 and virtio network devices
originally put in place before using libevent for async device
events. The vcpu thread doesn't need to process device data as it is
handled by the libevent thread.

This has the benefit of simplifying some of the message passing
between threads introduced to the ns8250 uart since both the vcpu
and libevent threads were processing read events.

No functional change intended. Tested by many, including abieber@,
weerd@, Mischa Peters, and Matthias Schmidt. (Thanks.)

OK mlarkin@