tests: qemuhotplugtest: Set the cpu version at source for PPC64 tests

tests: qemuhotplugtest: Set the cpu version at source for PPC64 tests

Commit 140ff3c5141 ("tests: qemuhotplugtest: Fix arch-specific parts of
'ppc64' test XMLs") hardcoded the CPU model as POWER9 in the test result
XMLs. However, this value actually reflects the host CPU model detected
at build or test time, and can vary depending on the machine where the
tests run.

As newer POWER CPU models (e.g., POWER10, POWER11) become common, this
requires continuous updates to the test result files to match the CPU
version detected on the host. This adds unnecessary maintenance effort.

Fix this by updating the test source domain XMLs to specify POWER9 (or
any fixed version) as the CPU model. This ensures the test result files
stay stable and do not require updates every time a newer CPU is used on
the host system.

Signed-off-by: Narayana Murty N <nnmlinux@linux.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>