qmp: object-add: Validate class before creating object

Currently it is very easy to crash QEMU by issuing an object-add command
using an abstract class or a class that doesn't support
TYPE_USER_CREATABLE as parameter.

Example: with the following QMP command:

    (QEMU) object-add qom-type=cpu id=foo

QEMU aborts at:

    ERROR:qom/object.c:335:object_initialize_with_type: assertion failed: (type->abstract == false)

This patch moves the check for TYPE_USER_CREATABLE before object_new(),
and adds a check to prevent the code from trying to instantiate abstract
classes.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Tested-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
