tests: do not mangle real qemu caps in xml2argvtest

tests: do not mangle real qemu caps in xml2argvtest

None of the things testUpdateQEMUCaps adjusts are applicable
for tests that use the DO_TEST_CAPS macros, i.e.
real QEMU capabilities parsed from the XML files:

The architecture must be chosen before we even open the caps
file, CPU models are already present and the expensive HostModel
computation was already done in virQEMUCapsLoadCache.

Introduce FLAG_REAL_CAPS and skip the whole testUpdateQEMUCaps
function for DO_TEST_CAPS.

This speeds up the test by 25 %

Signed-off-by: Ján Tomko <jtomko@redhat.com>