All defined properties of QemuConsole are mandatory and no access to them
should fail. Nevertheless not checking returned errors is bad because in case
of unexpected failure it will hide the bug and cause a memory leak.

Abort in case of unexpected property access errors. This change exposed a bug
where an attempt was made to write to a read-only property "head".

Set "head" property's value at creation time and do not attempt to change it
later. This fixes the bug mentioned above.

Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>