conf: Add check/error for domain supports cold/hotplug

conf: Add check/error for domain supports cold/hotplug

https://bugzilla.redhat.com/show_bug.cgi?id=1624336

Add a check during virDomainDefCompatibleDevice whether the
domain supports cold/hotplug of a memory module even though
this duplicates the qemuDomainDefValidateMemoryHotplug check.

Without this check, the cold/hot plug would fail on the
subsequent mem_memory check (since it's 0). Adding a check
for max_memory > 0 would allow the subsequent hotplug check
to fail, but would cause coldplug to fail with the somewhat
opaque message "no free memory device slot available".

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>