feat: add --kernel flag for QEMU system emulation mode

Add --kernel <PATH> option to boot extracted rootfs in a QEMU virtual
machine instead of namespace/chroot mode. The rootfs is converted to an
ext4 disk image using mke2fs and booted with the provided kernel.

Cargo.toml

@@ -35,6 +35,7 @@ anyhow = "1"
 
 # Utilities
 dirs = "6"
+which = "7"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "io-util"] }
 futures-util = "0.3"
 indicatif = "0.18"