qemu: Don't error out on 'unknown' memory model in qemuMonitorJSONGetMemoryDeviceInfo()

qemu: Don't error out on 'unknown' memory model in qemuMonitorJSONGetMemoryDeviceInfo()

When starting QEMU (or when reconnecting to a running one),
qemuMonitorJSONGetMemoryDeviceInfo() is called to refresh info on
memory devices. In here, query-memory-devices is called which
returns info on all memory devices. The result is then iterated
over and for some memory models runtime information is updated.
The rest is to be ignored. Except, when introducing SGX support,
this was turned into an error leaving us unable to start any
domain with virtio-pmem memory device (as virtio-pmem is to be
ignored).

Fixes: ddb1bc051959eef4ad7ed6ac47b57056632bdb5e
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>