virtio only support packed ring size power of 2

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1877015
Reported by 'xuan' on 2020-05-06 :

Issue discription：
When QEMU starts with "-device virtio-net-
pci,netdev=netdev0,mac=52:54:00:00:00:01,disable-
modern=false,mrg_rxbuf=on,rx_queue_size=1025,tx_queue_size=1025,mq=on,vectors=15,packed=on"

It raises error: Invalid rx_queue_size (= 1025), must be a power of 2
between 256 and 1024

Analysis:
According to virtio1.1 spec, the packed queue size value does not have to
be a power of 2.