qemu_monitor: Switch to virDomainMemoryModel enum in qemuMonitorJSONGetMemoryDeviceInfo()

qemu_monitor: Switch to virDomainMemoryModel enum in qemuMonitorJSONGetMemoryDeviceInfo()

When processing memory devices (as a reply from QEMU), a bunch of
STREQ()-s is used. Fortunately, the set of strings we process is
the same as virDomainMemoryModel enum. Therefore, we can use
virDomainMemoryModelTypeFromString() and then use integer
comparison (well, switch()). This has an upside: introducing a
new memory model lets us see what places need adjusting
immediately at compile time.

NB, this is in contrast with cmd line generator
(qemuBuildMemoryDeviceProps()), where more specific models are
generated (e.g. "pc-dimm", "virtio-mem-pci", etc.). But QEMU
reports back the parent model, instead of specific child
instance.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>