upgrade: unset XDG_RUNTIME_DIR before calling libguestfs
upgrade: unset XDG_RUNTIME_DIR before calling libguestfs

Newer (> 1.32 - RHEL 7.4) version of libguestfs relies on the value
of XDG_RUNTIME_DIR, if set, to choose its temporary directory.
Since we are changing from root user to vdsm user, we have to
unset XDG_RUNTIME_DIR as well.

Bug-Url: https://bugzilla.redhat.com/1467813

Change-Id: Icda24ad4cc381126ddc094377babf413457e5eb7
Signed-off-by: Simone Tiraboschi <cb03129c66ba4da7ba7044049d0f43907bf857bb@redhat.com>

oVirt/ovirt-hosted-engine-setup,oVirt/ovirt-hosted-engine-setup