testUpdateQEMUCaps: Don't leak host cpuData

When preparing qemuCaps for test cases the following is
happening:

qemuTestParseCapabilitiesArch() is called, which calls
virQEMUCapsLoadCache() which in turn calls
virQEMUCapsInitHostCPUModel() which sets qemuCaps->kvmCPU and
qemuCaps->tcgCPU.

But then the code tries to update the capabilities:

testCompareXMLToArgv() calls testUpdateQEMUCaps() which calls
virQEMUCapsInitHostCPUModel() again overwriting previously
allocated memory. The solution is to free host cpuData in
testUpdateQEMUCaps().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>