adds a new <hostdev managed='(yes|no)'> property to host devices in domains

adds a new <hostdev managed='(yes|no)'> property to host devices in domains
* docs/schemas/domain.rng src/domain_conf.c src/domain_conf.h
  src/qemu_conf.c
  tests/qemuxml2argvdata/qemuxml2argv-hostdev-pci-address.xml
  tests/qemuxml2argvdata/qemuxml2argv-hostdev-usb-address.xml
  tests/qemuxml2argvdata/qemuxml2argv-hostdev-usb-product.xml:
  adds a new <hostdev managed='(yes|no)'> property
  to host devices indicating whether or not we should
  automatically dettach/reset, patch by Mark McLoughlin
daniel