Extend v3 migration protocol to allow app supplied XML for target

This extends the v3 migration protocol such that the
virDomainMigrateBegin3 and virDomainMigratePerform3
methods accept an application supplied XML config for
the target VM.

If the 'xmlin' parameter is NULL, then Begin3 uses the
current guest XML as normal. A driver implementing the
Begin3 method should either reject all non-NULL 'xmlin'
parameters, or strictly validate that the app supplied
XML does not change guest ABI.

The Perform3 method also needed the xmlin parameter to
cope with the Peer2Peer migration sequence.

NB it is not yet possible to use this capability since
neither of the public virDomainMigrate/virDomainMigrateToURI
methods have a way to pass in XML.

* daemon/remote.c, src/remote/remote_driver.c,
  src/remote/remote_protocol.x, src/remote_protocol-structs:
  Add 'remote_string xmlin' parameter to begin3/perform3
  RPC messages
* src/libvirt.c, src/driver.h, src/libvirt_internal.h: Add
  'const char *xmlin' parameter to Begin3/Perform3 methods
* src/qemu/qemu_driver.c, src/qemu/qemu_migration.c,
  src/qemu/qemu_migration.h: Pass xmlin parameter around
  migration methods

src/remote_protocol-structs

@@ -1457,6 +1457,7 @@ struct remote_domain_get_state_ret {
 };
 struct remote_domain_migrate_begin3_args {
         remote_nonnull_domain      dom;
+        remote_string              xmlin;
         uint64_t                   flags;
         remote_string              dname;
         uint64_t                   resource;
@@ -1504,6 +1505,7 @@ struct remote_domain_migrate_prepare_tunnel3_ret {
 };
 struct remote_domain_migrate_perform3_args {
         remote_nonnull_domain      dom;
+        remote_string              xmlin;
         struct {
                 u_int              cookie_in_len;
                 char *             cookie_in_val;