virstoragefile: Introduce virStoragePRDef

virstoragefile: Introduce virStoragePRDef

This is a definition that holds information on SCSI persistent
reservation settings. The XML part looks like this:

  <reservations enabled='yes' managed='no'>
    <source type='unix' path='/path/to/qemu-pr-helper.sock' mode='client'/>
  </reservations>

If @managed is set to 'yes' then the <source/> is not parsed.
This design was agreed on here:

https://www.redhat.com/archives/libvir-list/2017-November/msg01005.html

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