Refine dependency on vm-memory

Refine dependency on vm-memory crate by:
1) relax version number constraint to "0.x.y"
2) enable feature vm-memory/backend-mmap for development only because
   it's used by unit test cases.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

Author: jiangliu

Cargo.toml

@@ -12,4 +12,7 @@ bzimage = []
 pe = []
 
 [dependencies]
-vm-memory = {version = ">=0.2.0", features = ["backend-mmap"]}
+vm-memory = ">=0.2.0"
+
+[dev-dependencies]
+vm-memory = {features = ["backend-mmap"]}