qemu: Shorten per-domain directory names

qemu: Shorten per-domain directory names

Per-domain directories were introduced in order to be able to
completely separate security labels for each domain (commit
f1f68ca33433825ce0deed2d96f1990200bc6618).  However when the domain
name is long (let's say a ridiculous 110 characters), we cannot
connect to the monitor socket because on length of UNIX socket address
is limited.  In order to get around this, let's shorten it in similar
fashion and in order to avoid conflicts, throw in an ID there as well.
Also save that into the status XML and load the old status XMLs
properly (to clean up after older domains).  That way we can change it
in the future.

The shortening can be seen in qemuxml2argv tests, for example in the
hugepages-pages2 case.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>