bhyve: Fix vq_getchain() error handling bugs in various device models

Reviewed by: grehan, khng
Approved by: so
Security: CVE-2021-29631
Security: FreeBSD-SA-21:13.bhyve

(cherry picked from commit 71fbc6faed62e8eb5864f7c40839740f5e9f5558)

Git Hash: eb01dfe6c01c700418a872ae520187a1292ea7da
Git Author: markj@FreeBSD.org

MFC r335025:

Add SPDX tags to bhyve(8).

Discussed with: rgrimes, pfg and mav.
Obtained from: TrueOS
Sponsored by: iXsystems Inc.

MFC 313727, 317483
In addition, replace the missing caph routines with
small helper functions (bhyverun.c) or an open-coded
replacement (uart_emul.c)

313727 Capsicumize bhyve
317483 Allow CAP_MMAP_RW on memfd for PCI passthru

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Set the subvendor field in config space to the vendor ID.
This is required by the Windows virtio drivers to correctly
match a device.

Submitted by: Leon Dang (ldang@nahannisys.com)
MFC after: 2 weeks

Modify virtqueue helpers added in r253440 to allow queuing.

Original virtqueue design allows queued and out-of-order processing, but
helpers added in r253440 suppose only direct blocking in-order one.
It could be fine for network, etc., but it is a huge limitation for storage
devices.

Add a callback to be notified about negotiated features.

Submitted by: luigi
Obtained from: Vincenzo Maffione, Universita` di Pisa
MFC after: 3 days

Use calloc() in favor of malloc + memset.

Reviewed by: neel

Add support for the virtio RNG entropy-source device.
Call through to /dev/random synchronously to fill
virtio buffers with RNG data.

Tested with FreeBSD-CURRENT and Ubuntu guests.

Submitted by: Leon Dang
Discussed with: markm
MFC after: 3 weeks
Sponsored by: Nahanni Systems