Fix EmbeddedInstances as instance properties.

Set correct CMPI type when setting string property with EmbeddedInstance
qualifier.

For example, a provider may set string property as embedded instance:

model['MyEmbeddedProperty'] = CIMInstance(...)

The CIMProperty object, which gets created for this property, will have:

    property.type = 'string'
    property.embedded_object = 'instance'

(see CIMProperty constructor).

In order to use the right CMPI type when converting this property to CMPI
data, we should consider also property.embedded_object and not only .type.

This patch changes resulting CMPI type from CMPI_string to CMPI_instance
for such embedded instances.