qemu_passt: Remove passt socket file on exit

qemu_passt: Remove passt socket file on exit

Just like it can't remove its own PID files, passt can't unlink its
own socket upon exit (unless the initialisation fails), because it
has no access to the filesystem at runtime.

Remove the socket file in qemuPasstKill().

Fixes: a56f0168d576 ("qemu: hook up passt config to qemu domains")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>