qemu: Check for presence of device and properities when getting CPUID

qemu: Check for presence of device and properities when getting CPUID

The QOM path in qemu that contains the CPUID registers of a running VM
may not be present (introduced in QEMU 1.5).

Since commit d94b7817719 we have a regression with QEMU that don't
support reporting of the CPUID register state via the monitor as the
process startup code expects the path to exist.

This patch adds code that checks with the monitor if the requested path
already exists and uses it only in this case.