virt plugin: Factor the read state into a struct

virt plugin: Factor the read state into a struct

The lv_read function needs some bookkeeping data to track which
domain, block interface and network interface should be polled
for new data.

This patch factors out this data, previously scattered as module
globals, in a new struct. This makes the code a little tidier
and more reusable.

Signed-off-by: Francesco Romani <fromani@redhat.com>