Introduces several new public API options for migration

 - VIR_MIGRATE_PEER2PEER: With this flag the client only
   invokes the virDomainMigratePerform method, expecting
   the source host driver to do whatever is required to
   complete the entire migration process.
 - VIR_MIGRATE_TUNNELLED: With this flag the actual data
   for migration will be tunnelled over the libvirtd RPC
   channel. This requires that VIR_MIGRATE_PEER2PEER is
   also set.
 - virDomainMigrateToURI: This is variant of the existing
   virDomainMigrate method which does not require any
   virConnectPtr for the destination host. Given suitable
   driver support, this allows for all the same modes as
   virDomainMigrate()

The URI for VIR_MIGRATE_PEER2PEER must be a valid libvirt
URI. For non-p2p migration a hypervisor specific migration
URI is used.

virDomainMigrateToURI without a PEER2PEER flag is only
support for Xen currently, and it involves XenD talking
directly to XenD, no libvirtd involved at all.

* include/libvirt/libvirt.h.in: Add VIR_MIGRATE_PEER2PEER
  flag for migration
* src/libvirt_internal.h: Add feature flags for peer to
  peer migration (VIR_FEATURE_MIGRATE_P2P) and direct
  migration (VIR_MIGRATE_PEER2PEER mode)
* src/libvirt.c: Implement support for VIR_MIGRATE_PEER2PEER
  and virDomainMigrateToURI APIs.
* src/xen/xen_driver.c: Advertise support for DIRECT migration
* src/xen/xend_internal.c: Add TODO item for p2p migration
* src/libvirt_public.syms: Export virDomainMigrateToURI
  method
* src/qemu/qemu_driver.c: Add support for PEER2PEER and
  migration, and adapt TUNNELLED migration.
* tools/virsh.c: Add --p2p and --direct args and use the
  new virDomainMigrateToURI method where possible.