Add "-u" option to bhyve(8) to indicate that the RTC should maintain UTC time.

The default remains localtime for compatibility with the original device model
in bhyve(8). This is required for OpenBSD guests which assume that the RTC
keeps UTC time.

Reviewed by: grehan
Pointed out by: Jason Tubnor (jason@tubnor.net)
MFC after: 2 weeks