When running a suid Xorg mkfifo and bind produce files owned by root.
Change the ownership so that the user launched vdagent & vdagentd can
write to them. This also makes it easier to cleanup the files from the
Xspice process that is not running as root (and is launching both
vdagent & vdagentd, hence they are not running as root either).

The patch adds two new parameters, uid and gid of the agent used files,
namely the virtio unix domain socket and the uinput fifo.