test_driver: introduce domain-private data

test_driver: introduce domain-private data

vm-specific data can be used by APIs that need to preserve some state
between calls

Some of them are:
- FS-related APIs for remembering which mountpoints are frozen
- virDomainSetTime / virDomainGetTime for maintaining time information
- virDomainSetIOThreadParams for storing the I/O thread parameters
- virDomainManagedSaveDefineXML for internally storing the VM definition

Signed-off-by: Ilias Stamatis <stamatis.iliass@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>