Revert "virtio: postpone the execution of event_notifier_cleanup function"

Revert "virtio: postpone the execution of event_notifier_cleanup function"

This reverts commit 4fe6d78b2e241f41208dfb07605aace4becfc747 as it is
reported to break cleanup and migration.

Cc: Gal Hammer <ghammer@redhat.com>
Cc: Sitong Liu <siliu@redhat.com>
Cc: Xiaoling Gao <xiagao@redhat.com>
Suggested-by: Greg Kurz <groug@kaod.org>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Reported-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>