draft: feat(balloon): add balloon device driver

TODO: explanatory commit message

Author: hcsch

Cargo.toml

@@ -69,6 +69,7 @@ trace = []
 udp = ["smoltcp", "smoltcp/socket-udp"]
 vga = []
 vsock = ["pci"]
+balloon = ["pci"]
 allocation-stats = ["talc/counters"]
 
 [lints.rust]

src/config.rs

@@ -7,7 +7,8 @@ pub(crate) const USER_STACK_SIZE: usize = 0x0010_0000;
 #[cfg(any(
 	all(any(feature = "tcp", feature = "udp"), not(feature = "rtl8139")),
 	feature = "fuse",
-	feature = "vsock"
+	feature = "vsock",
+	feature = "balloon"
 ))]
 pub(crate) const VIRTIO_MAX_QUEUE_SIZE: u16 = if cfg!(feature = "pci") { 2048 } else { 1024 };