migration: qmp_migrate(): keep working after syntax error

If a user or QMP client enter a bad syntax for the migrate
command in QMP/HMP, then the migrate command will never succeed
from that point on.

For example, if you enter:

(qemu) migrate tcp;0:4444
migrate: Parameter 'uri' expects a valid migration protocol

Then the migrate command will always fail from now on:

(qemu) migrate tcp:0:4444
migrate: There's a migration process in progress

The problem is that qmp_migrate() sets the migration status to
MIG_STATE_SETUP and doesn't reset it on syntax error. This bug
was introduced by commit 29ae8a4133082e16970c9d4be09f4b6a15034617.

Reviewed-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
