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>

vm-allocator/Cargo.toml

@@ -6,5 +6,5 @@ edition = "2018"
 
 [dependencies]
 libc = "0.2.104"
-vm-memory = "0.7.0"
+vm-memory = "0.6.0"
 arch = { path = "../arch" }