qmp: add block-job-pause and block-job-resume

Add QMP commands matching the functionality.

Paused jobs cannot be canceled without first resuming them.  This
ensures that I/O errors are never missed by management.  However, an
optional force argument can be specified to allow that.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>