Revert "migration: move only_migratable to MigrationState"

Revert "migration: move only_migratable to MigrationState"

This reverts commit 3df663e575f1876d7f3bc684f80e72fca0703d39.
This reverts commit b605c47b57b58e61a901a50a0762dccf43d94783.

Command line option --only-migratable is for disallowing any
configuration that can block migration.

Initially, --only-migratable set global variable @only_migratable.

Commit 3df663e575 "migration: move only_migratable to MigrationState"
replaced it by MigrationState member @only_migratable.  That was a
mistake.

First, it doesn't make sense on the design level.  MigrationState
captures the state of an individual migration, but --only-migratable
isn't a property of an individual migration, it's a restriction on
QEMU configuration.  With fault tolerance, we could have several
migrations at once.  --only-migratable would certainly protect all of
them.  Storing it in MigrationState feels inappropriate.

Second, it contributes to a dependency cycle that manifests itself as
a bug now.

Putting @only_migratable into MigrationState means its available only
after migration_object_init().

We can't set it before migration_object_init(), so we delay setting it
with a global property (this is fixup commit b605c47b57 "migration:
fix handling for --only-migratable").

We can't get it before migration_object_init(), so anything that uses
it can only run afterwards.

Since migrate_add_blocker() needs to obey --only-migratable, any code
adding migration blockers can run only afterwards.  This contributes
to the following dependency cycle:

* configure_blockdev() must run before machine_set_property()
  so machine properties can refer to block backends

* machine_set_property() before configure_accelerator()
  so machine properties like kvm-irqchip get applied

* configure_accelerator() before migration_object_init()
  so that Xen's accelerator compat properties get applied.

* migration_object_init() before configure_blockdev()
  so configure_blockdev() can add migration blockers

The cycle was closed when recent commit cda4aa9a5a0 "Create block
backends before setting machine properties" added the first
dependency, and satisfied it by violating the last one.  Broke block
backends that add migration blockers.

Moving @only_migratable into MigrationState was a mistake.  Revert it.

This doesn't quite break the "migration_object_init() before
configure_blockdev() dependency, since migrate_add_blocker() still has
another dependency on migration_object_init().  To be addressed the
next commit.

Note that the reverted commit made -only-migratable sugar for -global
migration.only-migratable=on below the hood.  Documentation has only
ever mentioned -only-migratable.  This commit removes the arcane &
undocumented alternative to -only-migratable again.  Nobody should be
using it.

Conflicts:
	include/migration/misc.h
	migration/migration.c
	migration/migration.h
	vl.c

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190401090827.20793-3-armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>