security: don't fail if built without attr support

If built without attr support removing any image will trigger
qemuBlockRemoveImageMetadata (the one that emits the warning)
-> qemuSecurityMoveImageMetadata
-> virSecurityManagerMoveImageMetadata
-> virSecurityDACMoveImageMetadata
-> virSecurityDACMoveImageMetadataHelper
-> virProcessRunInFork (spawns subprocess)
-> virSecurityMoveRememberedLabel

In there due to !HAVE_LIBATTR virFileGetXAttrQuiet will return
ENOSYS and from there the chain will error out.

That is wrong and looks like:
libvirtd[6320]: internal error: child reported (status=125):
libvirtd[6320]: Unable to remove disk metadata on vm testguest from
/var/lib/uvtool/libvirt/images/testguest.qcow (disk target vda)

This change makes virSecurityDACMoveImageMetadataHelper and
virSecuritySELinuxMoveImageMetadataHelper accept that
error code gracefully and in that sense it is an extension of:
5214b2f1a3f "security: Don't skip label restore on file systems lacking XATTRs"
which does the same for other call chains into the virFile*XAttr functions.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>