qemu-iotests: add infrastructure of fd passing via SCM

This patch make use of the compiled scm helper program to transfer
fd via unix socket at runtime.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>