vhost_user_block: Use config::OptionParser to simplify block backend parsing

Switch to using the recently added OptionParser in the code that parses
the block backend.

Fixes: #1092

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/config.rs

@@ -317,9 +317,9 @@ impl<'a> VmParams<'a> {
     }
 }
 
-struct Toggle(bool);
+pub struct Toggle(pub bool);
 
-enum ToggleParseError {
+pub enum ToggleParseError {
     InvalidValue(String),
 }