kvm tools: make virtio-net mq max queues configurable

This patch makes the maximum amount of vqs configurable. To use it pass a 'mq'
option to network device configuration. For example:

	vm run -n mode=tap,mq=4

Will allow up to 4 queue pairs for that network device.

Note that not specifiying mq, or setting mq=0 will disable virtio-net
multiqueuing.

Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>