Restore power-of-2 queue count constraint from r290948

When vmx(4) was converted to an iflib driver in r343291, the
power-of-2 queue count constraint was removed as it appeared that
current implementations of the VMXNET3 virtual device no longer
required that constraint. It turns out that some of the
implementations still do, and on such systems, the device will fail to
initialize when configured with a non-power-of-2 RX or TX queue count.

PR: 237321
Reported by: ncrogers@gmail.com
MFC after: 1 week