Support per-VM migration type selectors

Allow a VM to specify its migration type as a default choice. The valid
options are "default" (i.e. behave as now), "live" which forces a live
migration only, and "shutdown" which forces a shutdown migration only.
The new option is treated as a VM meta option and is set to default if
not found.

api-daemon/migrations/versions/3efe890e1d87_pvc_version_0_9_0.py

@@ -0,0 +1,28 @@
+"""PVC version 0.9.0
+
+Revision ID: 3efe890e1d87
+Revises: 3bc6117ea44d
+Create Date: 2020-10-29 11:49:58.756626
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '3efe890e1d87'
+down_revision = '3bc6117ea44d'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('system_template', sa.Column('migration_method', sa.Text(), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('system_template', 'migration_method')
+    # ### end Alembic commands ###