edu: fix memory leak on msi_broken platforms

If msi_init fails, the thread has already been created and the
mutex/condvar are not destroyed.  Initialize everything only
after the point where pci_edu_realize cannot fail.

Reported-by: Markus Armbruster <armbru@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>