build: bump bitflags from 1.2.1 to 1.3.1

Bumps [bitflags](https://github.com/bitflags/bitflags) from 1.2.1 to 1.3.1.
- [Release notes](https://github.com/bitflags/bitflags/releases)
- [Changelog](https://github.com/bitflags/bitflags/blob/main/CHANGELOG.md)
- [Commits](https://github.com/bitflags/bitflags/compare/1.2.1...1.3.1)

---
updated-dependencies:
- dependency-name: bitflags
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

vmm/Cargo.toml

@@ -19,7 +19,7 @@ acpi_tables = { path = "../acpi_tables", optional = true }
 anyhow = "1.0.42"
 arc-swap = "1.3.0"
 arch = { path = "../arch" }
-bitflags = "1.2.1"
+bitflags = "1.3.1"
 block_util = { path = "../block_util" }
 clap = "2.33.3"
 devices = { path = "../devices" }