qemu: migration: Preserve error across qemuDomainSetMaxMemLock() on error paths

qemu: migration: Preserve error across qemuDomainSetMaxMemLock() on error paths

When a VM terminates itself while it's being migrated in running state
libvirt would report wrong error:

 error: cannot get locked memory limit of process 2502057: No such file or directory

rather than the proper error:

 error: operation failed: domain is not running

Remember the error on error paths in qemuMigrationSrcConfirmPhase and
qemuMigrationSrcPerformPhase.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>