migration: don't use uninitialized variables

The qmp_migrate method uses the 'blk' and 'inc' parameter without
checking if they're valid or not (they may be uninitialized if
command is received via QMP)

Signed-off-by: Pawit Pornkitprasan <p.pawit@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
