Bugfix: CLOUDSTACK-6292 live migration failed because of VNC server address is not changed

Make MigrateKVMAsync pass domain xml to Domain#migrate method
make clarify what options are provided for Domain#migrate call
Implement DataMotionStoragegy for KVM
Bugfix: hypervisor version comparison
Update hypervisor_capabilities record of KVM

to indicate live storage migration support
Change hypervisor_version of KVM and LXC from NULL to 'default'

to match the value of hypervisor_capabilities record
Shape up MigrateCommand handler