There is no longer a need to expose the request and related APIs in
virtio-scsi.h since there are no callers outside virtio-scsi.c.

Note the block comment in VirtIOSCSIReq has been adjusted to meet the
coding style.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20220427143541.119567-7-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>