Hotplugging and hotunplugging char devices is only supported through
'-device' and the check for device capability should be independently.

Coverity also complains about 'tmpChr->info.alias' could be NULL and we
are dereferencing it but it somehow only in this case don't recognize
that the value is set by 'qemuAssignDeviceChrAlias' so it's clearly
false positive. Add sa_assert to make coverity happy.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>