Storage interfaces like virtio-blk can be configured with block size
information so that the guest can take advantage of efficient I/O
request sizes.

According to the SCSI Block Commands (SBC) standard a device's block
size is "almost always greater than one byte and may be a multiple of
512 bytes".  QEMU currently has a 512 byte minimum block size because
the block layer functions work at that granularity.  Furthermore, the
block size should be a power of 2 because QEMU calculates bitmasks from
the value.

Introduce a "blocksize" property type so devices can enforce these
constraints on block size values.  If the constraints are relaxed in the
future then this property can be updated.

Introduce the new PropertyValueNotPowerOf2 QError so QMP clients know
exactly why a block size value was rejected.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>