hw/net: fix vmxnet3 live migration

At some point vmxnet3 live migration stopped working and git-bisect
didn't help finding a working version.
The issue is the PCI configuration space is not being migrated
successfully and MSIX remains masked at destination.

Remove the migration differentiation between PCI and PCIe since
the logic resides now inside VMSTATE_PCI_DEVICE.
Remove also the VMXNET3_COMPAT_FLAG_DISABLE_PCIE based differentiation
since at 'realize' time is decided if the device is PCI or PCIe,
then the above macro is enough.

Use the opportunity to move to the standard VMSTATE_MSIX
instead of the deprecated SaveVMHandlers.

Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-Id: <20190705010711.23277-1-marcel.apfelbaum@gmail.com>
Tested-by: Sukrit Bhatnagar <skrtbhtngr@gmail.com>
Reviewed-by: Dmitry Fleytman <dmitry.fleytman@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>