maint: avoid 'const fooPtr' in tests

maint: avoid 'const fooPtr' in tests

'const fooPtr' is the same as 'foo * const' (the pointer won't
change, but it's contents can).  But in general, if an interface
is trying to be const-correct, it should be using 'const foo *'
(the pointer is to data that can't be changed).

Fix up offenders in the testsuite.

* tests/cputest.c (cpuTestCompareXML): Use intended type.
* tests/qemucapabilitiestest.c (testQemuCaps): Likewise.
* tests/qemumonitorjsontest.c: Drop const.

Signed-off-by: Eric Blake <eblake@redhat.com>