qemu: Create multipath targets for PRs

If a disk has persistent reservations enabled, qemu-pr-helper
might open not only /dev/mapper/control but also individual
targets of the multipath device. We are already querying for them
in CGroups, but now we have to create them in the namespace too.
This was brought up in [1].

1: https://bugzilla.redhat.com/show_bug.cgi?id=1711045#c61

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Tested-by: Lin Ma <LMa@suse.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>