qemuDomainBuildNamespace: Handle special file mount points

qemuDomainBuildNamespace: Handle special file mount points

https://bugzilla.redhat.com/show_bug.cgi?id=1459592

In 290a00e41d I've tried to fix the process of building a
qemu namespace when dealing with file mount points. What I
haven't realized then is that we might be dealing not with just
regular files but also special files (like sockets). Indeed, try
the following:

1) socat unix-listen:/tmp/soket stdio
2) touch /dev/socket
3) mount --bind /tmp/socket /dev/socket
4) virsh start anyDomain

Problem with my previous approach is that I wasn't creating the
temporary location (where mount points under /dev are moved) for
anything but directories and regular files.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>