virttest.qemu_vm: Fix coding style

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.qemu_vm: Fix error in string repr of MigrateError

The problem was that the arguments for the string interpolation
were not passed as a tuple, as they should. When hitting
an unknown migration protocol error, the return would raise
a TypeError. Let's fix it.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>