bhyve: virtio-net: disable receive until features are negotiated

This patch fixes a race condition where the receive callback is called
while the device is being reset. Since the rx_merge variable may change
during reset, the receive callback may operate inconsistently with what
the guest expects.
Also, get rid of the unused rx_vhdrlen variable.

PR: 242023
Reported by: aleksandr.fedorov@itglobal.com
Reviewed by: markj, jhb
MFC with: r354552
Differential Revision: https://reviews.freebsd.org/D22440