migration-test: Add bootfile_create/delete() functions

The bootsector code is read only from the guest (otherwise we are
going to have problems with it being read from both source and
destination).

Create a single copy for all the tests.

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20230608224943.3877-10-quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>