Commit f506a4c1 changed virSetUIDGID() to be a noop
when uid/gid are -1, while it used to be a noop when
they are <= 0.

The changes in this commit broke creating new VMs in GNOME Boxes
as qemuDomainCheckDiskPresence gets called during domain creation/startup,
which in turn calls virFileAccessibleAs which fails after calling
virSetUIDGID(0, 0) (Boxes uses session libvirtd). virSetUIDGID is called with
(0, 0) as these are the default user/group values in virQEMUDriverConfig
for session libvirtd.

This commit changes virQEMUDriverConfigNew to use -1 as the unpriviledged
uid/gid. I've also looked at the various places where cfg->user is used,
and they all seem to handle -1 correctly.