viruuid: Rework virUUIDIsValid()

viruuid: Rework virUUIDIsValid()

The only test we do when checking for UUID validity is that
whether all bytes are the same (invalid UUID) or not (valid
UUID). The algorithm we use is needlessly complicated.

Also, the checked UUID is not modified and hence the argument can
be of 'const' type.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Han Han <hhan@redhat.com>