tools/virtio: compile with -pthread

When using pthreads, one has to compile and link with -lpthread,
otherwise e.g. glibc is not guaranteed to be reentrant.

This replaces -lpthread.

Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>