migration/savevm: remove duplicate check of migration_is_blocked

migration/savevm: remove duplicate check of migration_is_blocked

Current call flow of save_snapshot is:

  save_snapshot
    migration_is_blocked
      qemu_savevm_state
        migration_is_blocked

Since qemu_savevm_state is only called in save_snapshot, this means
migration_is_blocked has been already checked.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190424004700.12766-2-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>