migration (postcopy): move bdrv_invalidate_cache_all of of coroutine context

migration (postcopy): move bdrv_invalidate_cache_all of of coroutine context

There is a possibility to hit an assert in qcow2_get_specific_info that
s->qcow_version is undefined. This happens when VM in starting from
suspended state, i.e. it processes incoming migration, and in the same
time 'info block' is called.

The problem is that qcow2_invalidate_cache() closes the image and
memset()s BDRVQcowState in the middle.

The patch moves processing of bdrv_invalidate_cache_all out of
coroutine context for postcopy migration to avoid that. This function
is called with the following stack:
  process_incoming_migration_co
  qemu_loadvm_state
  qemu_loadvm_state_main
  loadvm_process_command
  loadvm_postcopy_handle_run

Signed-off-by: Denis V. Lunev <den@openvz.org>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Amit Shah <amit.shah@redhat.com>
Message-Id: <1456304019-10507-3-git-send-email-den@openvz.org>
Signed-off-by: Amit Shah <amit.shah@redhat.com>