bhyve: support noVNC SetPixelFormat request

The bhyve VNC server would ignore the SetPixelFormat message from the
VNC client. This change supports a limited implementation to detect
and reorder the colors such as requested from the noVNC client.

PR: 280984
Reviewed by: corvink
Differential Revision: https://reviews.freebsd.org/D46402
MFC after: 3 weeks