This is a contunuation of thread: 
  "lxc: truncate LOOP_GET_STATUS64.lo_file_name for long loop backing paths"
  https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/thread/NSOQDVNLF4XP34KBOGEACJYBVRGDXHQD/
with updated and more relevant subject.

This patch fixes problem reported as https://gitlab.com/libvirt/libvirt/-/work_items/63
but before it could be posted, it required fixing or rather implementing
aliase for block devices and file systems on LXC domains, commit
065337dd2c1b120be92a4960d0566eb745e8e350 and the patch posted here:
https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/thread/RWKBBQHE377G55CKFEX6DNZUXO6OJXAA/

Now, with the working aliases for for block devices and file systems, this change
implement the this naming pattern "libvirt-$UUID-$DEVALIAS" for LOOP_GET_STATUS64.lo_file_name
which is shown in output of the losetup.

Example, for domain with these file systems attached, where the second one
is backed by path with long file name:

    <filesystem type='file' accessmode='passthrough'>
      <driver type='loop' format='raw'/>
      <source file='/var/tmp/short.raw'/>
      <target dir='/short'/>
      <alias name='fs1'/>
    </filesystem>
    <filesystem type='file' accessmode='passthrough'>
      <driver type='loop' format='raw'/>
      <source file='/var/tmp/looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong.raw'/>
      <target dir='/long'/>
      <alias name='fs2'/>
    </filesystem>

The losetup output looks like below:

[root@rvm08m-3 ]# losetup -l -O +REF
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                                                                                                               DIO LOG-SEC REF
/dev/loop1         0      0         1  0 /var/tmp/looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong.raw   0     512 libvirt-942c6580-8c49-4b8d-8e79-bba79e9709fd-fs2
/dev/loop0         0      0         1  0 /var/tmp/short.raw                                                                                                        0     512 libvirt-942c6580-8c49-4b8d-8e79-bba79e9709fd-fs1

The last column is where the LOOP_GET_STATUS64.lo_file_name can be seen.

This patch does not add support for custom, user specified aliases because:
 1. At the start of the domain, user aliases are silently dropped during XML
    parsing and are auto-generated as fs0, fs1, ...
    VIR_DOMAIN_DEF_FEATURE_USER_ALIAS is not set for LXC domains.
 2. Attaching, hot-plug of a file system to live LXC domain is not supported:
    "error: unsupported configuration: device type 'filesystem' cannot be attached"