qcow_utils: add qcow feature to Cargo.toml

When the qcow2 support was moved behind a Cargo feature flag
(--features=qcow), the qcow_utils crate had some of its code modified to
check for `#[cfg(feature = "qcow")]`, but this feature was not actually
present in its Cargo.toml file, so enabling it had no effect.

This can be verified by checking the libqcow_utils.so contains a
create_qcow_with_size() function that does more than just return
-EINVAL.

BUG=None
TEST=emerge-brya crosvm # verify libqcow_utils.so via disassembly

Fixes: 8847de2725 ("qcow: Turn QCOW support into a feature")
Change-Id: I9416908c97d770a6e75324626124af482e6473b2
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4004348
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Reviewed-by: Christian Blichmann <cblichmann@google.com>
Commit-Queue: Christian Blichmann <cblichmann@google.com>

qcow_utils/Cargo.toml

@@ -8,6 +8,10 @@ edition = "2021"
 path = "src/qcow_utils.rs"
 crate-type = ["cdylib"]
 
+[features]
+default = ["qcow"]
+qcow = []
+
 [dependencies]
 libc = "*"
 disk = { path = "../disk" }