vhost-user-test: Use libqos instead of pxe-virtio.rom

vhost-user-test relies on iPXE just to initialize the virtio-net
device, and doesn't do any actual packet tx/rx testing.

In addition to that, the test relies on TCG, which is
imcompatible with vhost. The test only worked by accident: a bug
the memory backend initialization made memory regions not have
the DIRTY_MEMORY_CODE bit set in dirty_log_mask.

This changes vhost-user-test to initialize the virtio-net device
using libqos, and not use TCG nor pxe-virtio.rom.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
