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

(cherry picked from commit e9806d21282d3d5486b99b5c7062b490b00d8323)