vhost_user_*: mark --*-backend as required

I don't think the min_values(1) was doing anything, since
takes_value(true) was already set, and after this change I still get
an error if I try to supply the argument with no value.

But the unwrap below would fail if the argument wasn't supplied at
all.  By setting required(true) we get a nice message from clap
instead.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

vhost_user_block/src/main.rs

@@ -25,7 +25,7 @@ fn main() {
                 .long("block-backend")
                 .help(vhost_user_block::SYNTAX)
                 .takes_value(true)
-                .min_values(1),
+                .required(true),
         )
         .get_matches();
 

vhost_user_net/src/main.rs

@@ -22,7 +22,7 @@ fn main() {
                 .long("net-backend")
                 .help(vhost_user_net::SYNTAX)
                 .takes_value(true)
-                .min_values(1),
+                .required(true),
         )
         .get_matches();