bhyve: avoid resource leak in error path

In e820_finalize(), the e820_fwcfg_item variable, containing the
etc/e820 file (for the e820 table from the BIOS) is not free()'d when it
could not be added to the QEMU firmware configuration device (fw_cfg).

Reported by: Coverity Scan
CID: 1522761
Reviewed by: corvink, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45223

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

bhyve: Move more amd64-specific code under amd64/

mptable and the e820 are both rather amd64-specific and can be moved
easily.

In the case of e820, move the registration with qemu_fwcfg into e820.c,
as it simplifies bhyverun.c a bit and I can't see any downsides.

No functional change intended.

Reviewed by: corvink, jhb, emaste
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40552