Adjust if_vmx default receive parameters for better out-of-box performance

These adjustments improve performance with jumbo frames and/or LRO
enabled (i.e., when there may be multiple descriptors per packet) by
increasing the default size of the receive queues and by always using
page-sized buffers for the body type receive ring.

This patch also adjust the initialization of the max frame size to
remove cases where certain configuration sequences would result in 2K
receive buffers being used instead of 4K ones when jumbo frames were
enabled.

Reviewed by: gallatin
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23950