vm-migration: optimize dirty bitmap scanning

Adding itertools as dependency improves the iteration code in the
following significantly.

With this change, we don't need a copy of the vector. Just something
that can be coerced into an iterator. We also use the bit position
iterator to make the code somewhat clearer. The new code is much
faster, because it will not iterate over every bit, just each 1 bit in
the input.

The next commit will complete this optimization and have some concrete
numbers.

On-behalf-of: SAP julian.stecklina@sap.com
Signed-off-by: Julian Stecklina <julian.stecklina@cyberus-technology.de>

Cargo.lock

@@ -597,6 +597,12 @@ dependencies = [
  "windows-sys 0.61.0",
 ]
 
+[[package]]
+name = "either"
+version = "1.15.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "48c757948c5ede0e46177b7add2e67155f70e33c07fea8284df6576da70b3719"
+
 [[package]]
 name = "endi"
 version = "1.1.0"
@@ -1031,6 +1037,15 @@ version = "1.70.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7943c866cc5cd64cbc25b2e01621d07fa8eb2a1a23160ee81ce38704e97b8ecf"
 
+[[package]]
+name = "itertools"
+version = "0.14.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2b192c782037fadd9cfa75548310488aabdbf3d2da73885b31bd0abd03351285"
+dependencies = [
+ "either",
+]
+
 [[package]]
 name = "itoa"
 version = "1.0.15"
@@ -2418,6 +2433,7 @@ name = "vm-migration"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "itertools",
  "serde",
  "serde_json",
  "thiserror 2.0.17",