qemuDomainGetPreservedMounts: Fix suffixes for corner cases

qemuDomainGetPreservedMounts: Fix suffixes for corner cases

https://bugzilla.redhat.com/show_bug.cgi?id=1431112

Imagine a FS mounted on /dev/blah/blah2. Our process of creating
suffix for temporary location where all the mounted filesystems
are moved is very simplistic. We want:

/var/run/libvirt/qemu/$domName.$suffix\

were $suffix is just the mount point path stripped of the "/dev/"
prefix. For instance:

/var/run/libvirt/qemu/fedora.mqueue  for /dev/mqueue
/var/run/libvirt/qemu/fedora.pts     for /dev/pts

and so on. Now if we plug /dev/blah/blah2 into the example we see
some misbehaviour:

/var/run/libvirt/qemu/fedora.blah/blah2

Well, misbehaviour if /dev/blah/blah2 is a file, because in that
case we call virFileTouch() instead of virFileMakePath().
The solution is to replace all the slashes in the suffix with say
dots. That way we don't have to care about nested directories.
IOW, the result we want for given example is:

/var/run/libvirt/qemu/fedora.blah.blah2

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