Libvirt already has two types of panic devices - pvpanic and pSeries firmware.
This patch introduces the 'model' attribute and a new type of panic device.

'isa' model is for ISA pvpanic device.
'pseries' model is a default value for pSeries guests.
'hyperv' model is the new type. It's used for Hyper-V crash.

Schema and docs are updated for the new attribute.