vm: drop unused exception

The _MigrationError exception is defined and never used;
The close relative MigrationError (without the leading underscore) is.

This patch drops the unused _MigrationError.

Change-Id: I5af349e578ef9e5de95a1fe96ebaa7c34f2a6e77
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23901
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>