tools/xl: Fix build error following c/s f52fbcf7

tools/xl: Fix build error following c/s f52fbcf7

CentOS7 complains that 'ret' might be unused, and indeed this is the case for
`xl psr-hwinfo --cat`.

The logic for selecting which information to print was rather awkward.
Introduce a new 'all' which default to true, and is cleared if specific
options are selected.  This allows for a far more clear logic when choosing
whether to print information or not.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Chao Peng <chao.p.peng@linux.intel.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Chao Peng <chao.p.peng@linux.intel.com>