qemuDomainAttachDeviceMknodRecursive: Support file mount points

qemuDomainAttachDeviceMknodRecursive: Support file mount points

https://bugzilla.redhat.com/show_bug.cgi?id=1462060

Just like in the previous commit, when attaching a file based
device which has its source living under /dev (that is not a
device rather than a regular file), calling mknod() is no help.
We need to:

1) bind mount device to some temporary location
2) enter the namespace
3) move the mount point to desired place
4) umount it in the parent namespace from the temporary location

At the same time, the check in qemuDomainNamespaceSetupDisk makes
no longer sense. Therefore remove it.

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