qemu: Keep migration job active after failed post-copy

qemu: Keep migration job active after failed post-copy

When post-copy migration fails, we can't just abort the migration and
resume the domain on the source host as it is already running on the
destination host and no host has a complete state of the domain memory.
Instead of the current approach of just marking the domain on both ends
as paused/running with a post-copy failed sub state, we will keep the
migration job active (even though the migration API will return failure)
so that the state is more visible and we can better control what APIs
can be called on the domains and even allow for resuming the migration.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>