Implement some missing domain xml attrs/elems

[3442]

This PR adds support for the following in domains:

• vcpu.cpuset
• features.apic.eoi
• features.pae
• features.pvspinlock
• devices.disk.model
• devices.disk.snapshot
• devices.disk.source.index
• devices.disk.driver.io
• devices.hostdev.driver
• devices.interface.target
• devices.vsock

[AshleyYakeley]

Thanks.