virtio: fix queue size and number of descriptors

virtio: fix queue size and number of descriptors

The virtual queue ring size and the number of slots actually usable
are separate parameters. In the most common environment (QEMU)
the virtual queue ring size is 256, but some environments the
ring maybe much larger.

The ring size comes from the host and the driver must use the
actual size passed.

The number of descriptors can be either zero to use the whole
available ring, or some value smaller. This is used to limit
the number of mbufs allocated for the receive ring. If more
descriptors are requested than available the size is silently
truncated.

Note: the ring size (from host) must be a power of two, but
the number of descriptors used can be any size from 1 to the
size of the virtual ring.

Fixes: d78deadae4dc ("virtio: fix ring size negotiation")

Reported-by: Changchun Ouyang <changchun.ouyang@intel.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Changchun Ouyang <changchun.ouyang@intel.com>