cpr: reboot mode

Add the cpr-reboot migration mode.  Usage:

$ qemu-system-$arch -monitor stdio ...
QEMU 8.1.50 monitor - type 'help' for more information
(qemu) migrate_set_capability x-ignore-shared on
(qemu) migrate_set_parameter mode cpr-reboot
(qemu) migrate -d file:vm.state
(qemu) info status
VM status: paused (postmigrate)
(qemu) quit

$ qemu-system-$arch -monitor stdio -incoming defer ...
QEMU 8.1.50 monitor - type 'help' for more information
(qemu) migrate_set_capability x-ignore-shared on
(qemu) migrate_set_parameter mode cpr-reboot
(qemu) migrate_incoming file:vm.state
(qemu) info status
VM status: running

In this mode, the migrate command saves state to a file, allowing one
to quit qemu, reboot to an updated kernel, and restart an updated version
of qemu.  The caller must specify a migration URI that writes to and reads
from a file.  Unlike normal mode, the use of certain local storage options
does not block the migration, but the caller must not modify guest block
devices between the quit and restart.  To avoid saving guest RAM to the
file, the memory backend must be shared, and the @x-ignore-shared migration
capability must be set.  Guest RAM must be non-volatile across reboot, such
as by backing it with a dax device, but this is not enforced.  The restarted
qemu arguments must match those used to initially start qemu, plus the
-incoming option.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <1698263069-406971-6-git-send-email-steven.sistare@oracle.com>