gve: Add feature to adjust RX/TX queue counts

This change introduces new sysctl handlers that allow the user to change
RX/TX queue counts. As before, the default queue counts will be the max
value the device can support. When chaning queue counts, the interface turns
down momentarily while allocating/freeing resources as necessary.

Signed-off-by: Vee Agarwal <veethebee@google.com>

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49427