tests: Properly reset mocked CPU model

tests: Properly reset mocked CPU model

When switching the host architecture to something for which we do not
have any host CPU model defined, the mocked
virQEMUCapsProbeHostCPUForEmulator would just return the previous CPU
model resulting in strange combinations, such as "core2duo" host CPU
model in QEMU capabilities for "AArch64" architecture. It currently
doesn't break any test case, but we should fix it anyway to avoid future
surprises which would be quite hard to debug.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>