examples/vhost: clean switch worker

switch_worker() is the last piece of code that is messy yet it touches
virtio/vhost device.

Here do a cleanup, so that we will be less painful for later vhost ABI
refactoring.

The cleanup is straight forward: break long lines, move some code into
functions. The last, comment a bit on switch_worker().

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>