implement managedsave in libvirt xen legacy driver

implement managedsave in libvirt xen legacy driver

Implement the domainManagedSave, domainHasManagedSaveImage, and
domainManagedSaveRemove functions in the libvirt legacy xen driver.

domainHasManagedSaveImage check the managedsave image from filesystem
everytime. This is different from qemu and libxl driver. In qemu or
libxl driver, there is a hasManagesSave flag in virDomainObjPtr which
is not used in xen legacy driver. This flag could not add into xen
driver ptr either, because the driver ptr will be released at the end of
every libvirt api call. Meanwhile, AFAIK, xen store all the flags in
xen not in libvirt xen driver. There is no need to add this flag in xen.

Signed-off-by: Bamvor Jian Zhang <bjzhang@suse.com>