Parallels driver is 'stateless', like vmware or openvz drivers.
It collects information about domains during startup using
command-line utility prlctl. VMs in Parallels are identified by UUIDs
or unique names, which can be used as respective fields in
virDomainDef structure. Currently only basic info, like
description, virtual cpus number and memory amount, is implemented.
Querying devices information will be added in the next patches.

Parallels doesn't support non-persistent domains - you can't run
a domain having only disk image, it must always be registered
in system.

Functions for querying domain info have been just copied from
test driver with some changes - they extract needed data from
previously created list of virDomainObj objects.
Signed-off-by: NDmitry Guryanov <dguryanov@parallels.com>