Right now for xattr remapping, we support types of "prefix", "ok" or
"bad". Type "bad" returns -EPERM on setxattr() and hides xattr in
listxattr(). For getxattr(), mapping code returns -EPERM but
getxattr() code converts it to -ENODATA.

A new semantics is needed where if an xattr is unsupported, then
getxattr()/setxattr() return -ENOTSUP and listxattr() should hide
the xattr. This is needed to simulate that security.selinux is not
supported by virtiofs filesystem and in that case client falls back
to some default label specified by policy.

So add a new type "unsupported" which returns -ENOTSUP on getxattr()
and setxattr() and hides xattrs in listxattr().

For example, one can use following mapping rule to not support
security.selinux xattr and allow others.

"-o xattrmap=/unsupported/all/security.selinux/security.selinux//ok/all///"

Signed-off-by: German Maglione <gmaglione@redhat.com>