Revert "build: bump vm-memory from 0.6.0 to 0.7.0"

This was causing some issues because of the use of 2 different versions
for the vm-memmory crate. We'll wait for all dependencies to be properly
resolved before we move to 0.7.0.

This reverts commit 76b6c62d07.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

Cargo.toml

@@ -28,7 +28,7 @@ signal-hook = "0.3.10"
 thiserror = "1.0.30"
 vmm = { path = "vmm" }
 vmm-sys-util = "0.9.0"
-vm-memory = "0.7.0"
+vm-memory = "0.6.0"
 
 [build-dependencies]
 clap = { version = "2.33.3", features = ["wrap_help"] }