bhyve: Replace many fprintf(stderr, ...) calls with EPRINTLN

EPRINTLN handles newlines appropriately when stdout/stderr have been
reused as the backend for a serial port.

For bhyverun.c itself, the rule this attempts to follow is to use
regular fprintf/perror/warn/err prior to init_pci() (which is when
serial ports are configured) and to switch to EPRINTLN afterwards.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D42182