migrate: resume initially running VM when failing after convergence

migrate: resume initially running VM when failing after convergence

When phase2() is aborted after the migration already converged, then
after migrate_cancel, the VM might be in POSTMIGRATE state.

(There also is a conditional for SHUTDOWN state in QEMU's
migration_iteration_finish(), so it's likely possible to end up there
if the VM is shut down at the right time during migration, but no need
to resume then).

Detect the POSTMIGRATE state and resume the VM if it wasn't paused at
the beginning of the migration. There is no direct way to go to
PAUSED, so just print an error if the VM was paused at the beginning
of the migration.

Reported-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>