Add ability to set a default driver name/type when parsing disks

Add ability to set a default driver name/type when parsing disks

Record a default driver name/type in capabilities struct. Use this
when parsing disks if value is not set in XML config.

* src/conf/capabilities.h: Record default driver name/type for disks
* src/conf/domain_conf.c: Fallback to default driver name/type
  when parsing disks
* src/qemu/qemu_driver.c: Set default driver name/type to raw