net_pkt: Name vmxnet3 packet abstractions more generic

This patch drops "vmx" prefix from packet abstractions names
to emphasize the fact they are generic and not tied to any
specific network device.

These abstractions will be reused by e1000e emulation implementation
introduced by following patches so their names need generalization.

This patch (except renamed files, adjusted comments and changes in MAINTAINTERS)
was produced by:

git grep -lz 'vmxnet_tx_pkt' | xargs -0 perl -i'' -pE "s/vmxnet_tx_pkt/net_tx_pkt/g"
git grep -lz 'vmxnet_rx_pkt' | xargs -0 perl -i'' -pE "s/vmxnet_rx_pkt/net_rx_pkt/g"
git grep -lz 'VmxnetTxPkt' | xargs -0 perl -i'' -pE "s/VmxnetTxPkt/NetTxPkt/g"
git grep -lz 'VMXNET_TX_PKT' | xargs -0 perl -i'' -pE "s/VMXNET_TX_PKT/NET_TX_PKT/g"
git grep -lz 'VmxnetRxPkt' | xargs -0 perl -i'' -pE "s/VmxnetRxPkt/NetRxPkt/g"
git grep -lz 'VMXNET_RX_PKT' | xargs -0 perl -i'' -pE "s/VMXNET_RX_PKT/NET_RX_PKT/g"
sed -ie 's/VMXNET_/NET_/g' hw/net/vmxnet_rx_pkt.c
sed -ie 's/VMXNET_/NET_/g' hw/net/vmxnet_tx_pkt.c

Signed-off-by: Dmitry Fleytman <dmitry.fleytman@ravellosystems.com>
Signed-off-by: Leonid Bloch <leonid.bloch@ravellosystems.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>