Use the arch default transport method for network

lkvm by default sets up a virtio-pci transport for network, if none is
specified. This can be a problem on archs (e.g ARM64), where virtio-pci is
not supported yet and cause the following warning at exit.

  # KVM compatibility warning.
	virtio-net device was not detected.

This patch changes it to make use of the default transport method for the
architecture when none is specified. This will ensure that on every arch
we get the network up by default in the VM.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>