Allow character devices to have different target types

Allow character devices to have different target types

A character device's target (it's interface in the guest) had only a
single property: port. This patch is in preparation for adding targets
which require other properties.
Since this changes the conf type for character devices this affects
a number of drivers:

* src/conf/domain_conf.[ch] src/esx/esx_vmx.c src/qemu/qemu_conf.c
  src/qemu/qemu_driver.c src/uml/uml_conf.c src/uml/uml_driver.c
  src/vbox/vbox_tmpl.c src/xen/xend_internal.c src/xen/xm_internal.c:
  target properties are moved into a union in virDomainChrDef, and a
  targetType field is added to identify which union member should be
  used. All current code which touches a virDomainChrDef is updated both
  to use the new union field, and to populate targetType if necessary.