testutilsqemu: Write getter/setter for CPU def global variables

testutilsqemu: Write getter/setter for CPU def global variables

As of 47503cc859 we are statically linking libtest_utils_qemu.a
into qemuhotplugmock.so (see the original commit for reasoning).
However, this breaks ASAN on older clang because now
qemuhotplugtest has two instances of virCPUDef global variables
(cpuDefault, cpuHaswell, cpuPower8, cpuPower9). One that comes
from the binary itself (which also links with
libtest_utils_qemu.a) and the other from the mock. Resolve this
by making the variables static and introducing getter and setter.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>