The default console type may vary based on the OS type. ie a Xen
paravirt guests wants a 'xen' console, while a fullvirt guests
wants a 'serial' console.

A plain integer default console type in the capabilities does
not suffice. Instead introduce a callback that is passed the
OS type.

* src/conf/capabilities.h: Use a callback for default console
  type
* src/conf/domain_conf.c, src/conf/domain_conf.h: Use callback
  for default console type. Add missing LXC/OpenVZ console types.
* src/esx/esx_driver.c, src/libxl/libxl_conf.c,
  src/lxc/lxc_conf.c, src/openvz/openvz_conf.c,
  src/phyp/phyp_driver.c, src/qemu/qemu_capabilities.c,
  src/uml/uml_conf.c, src/vbox/vbox_tmpl.c,
  src/vmware/vmware_conf.c, src/xen/xen_hypervisor.c,
  src/xenapi/xenapi_driver.c: Set default console type callback