block: enforce constraints on block size properties

Nicolae Mogoreanu <mogo@google.com> noticed that I/O requests can lead
to QEMU crashes when the logical_block_size property is smaller than 512
bytes.

Using the new "blocksize" property we can properly enforce constraints
on the block size such that QEMU's block layer is able to operate
correctly.

Reported-by: Nicolae Mogoreanu <mogo@google.com>
Reported-by: Michael Halcrow <mhalcrow@google.com>
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>
