qemu: track 'cancelling' migration state

qemu: track 'cancelling' migration state

In qemu 2.3, the migration status will include 'cancelling' in the
window between when an asynchronous cancel has been requested and
when the migration is actually halted.  Previously, qemu hid this
state and reported 'active'.  Libvirt manages the sequence okay
even when the string is unrecognized (that is, it will report an
unknown state:

Migration: [ 69 %]^Cerror: internal error: unexpected migration status in cancelling.

but the migration is still cancelled), but recognizing the string
makes for a smoother user experience.

* src/qemu/qemu_monitor.h
(QEMU_MONITOR_MIGRATION_STATUS_CANCELLING): Add enum.
* src/qemu/qemu_monitor.c (qemuMonitorMigrationStatus): Map it.
* src/qemu/qemu_migration.c (qemuMigrationUpdateJobStatus): Adjust
clients.
* src/qemu/qemu_monitor_json.c
(qemuMonitorJSONGetMigrationStatusReply): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>