retrieve paused/running state at migration start

retrieve paused/running state at migration start

This patch fixes the bug where paused/running state is not
transmitted during migration.  As a result, in the QEMU driver
for example the machine was always started on the destination
end.

In order to do so, just read the state and if it is appropriate and
set the VIR_MIGRATE_PAUSED flag.

* src/libvirt.c (virDomainMigrateVersion1, virDomainMigrateVersion2):
  Automatically add VIR_MIGRATE_PAUSED when appropriate.
* src/xen/xend_internal.c (xenDaemonDomainMigratePerform): Give a nicer
  error message when migration of paused domains is attempted.