bhyve/virtio: check negotiated_caps for indirect descriptor support

vq_getchain() erroneously checked vc_hv_caps for indirect descriptor
support when it encountered an indirect descriptor. vc_hv_caps is used
in feature negotiation to advertise what features our device emulation
supports, but we should really check what features we have negotiated
with the driver.

Reviewed by: corvink
Differential Revision: https://reviews.freebsd.org/D53465