vhost-user-test: added proper TestServer *dest initialization in test_migrate()

vhost-user-test: added proper TestServer *dest initialization in test_migrate()

server->bus in _test_server_free() could be NULL, since TestServer
*dest in test_migrate() was not properly initialized like TestServer *s.
Added init_virtio_dev(dest) and uninit_virtio_dev(dest), so the fields
are properly set and when test_server_free(dest); is called, they can
be correctly freed.

The reason for that is init_virtio_dev() calls qpci_init_pc(), that
creates a QPCIBusPC * (returned as QPCIBus *), while test_server_free()
calls qpci_free_pc(), that frees the QPCIBus *. Not calling
init_virtio_dev() would leave the QPCIBus * of TestServer unset.

Problem came out once I modified  pci-pc.c and pci-pc.h, modifying
QPCIBusPC by adding another field before QPCIBus bus. Re-running the
tests showed vhost-user-test failing.

Signed-off-by: Emanuele Giuseppe Esposito <e.emanuelegiuseppe@gmail.com>
Message-Id: <1530022733-29581-1-git-send-email-esposem@usi.ch>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>