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.lock

@@ -310,10 +310,17 @@ dependencies = [
  "tempfile",
  "tokio",
  "users",
+ "which",
  "xz2",
  "zstd",
 ]
 
+[[package]]
+name = "either"
+version = "1.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91622ff5e7162018101f2fea40d6ebf4a78bbe5a49736a2020649edf9693679e"
+
 [[package]]
 name = "encode_unicode"
 version = "1.0.0"
@@ -329,6 +336,12 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "env_home"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7f84e12ccf0a7ddc17a6c41c93326024c42920d7ee630d04950e6926645c0fe"
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -1997,6 +2010,18 @@ dependencies = [
  "rustls-pki-types",
 ]
 
+[[package]]
+name = "which"
+version = "7.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "24d643ce3fd3e5b54854602a080f34fb10ab75e0b813ee32d00ca2b44fa74762"
+dependencies = [
+ "either",
+ "env_home",
+ "rustix",
+ "winsafe",
+]
+
 [[package]]
 name = "winapi-util"
 version = "0.1.11"
@@ -2197,6 +2222,12 @@ version = "0.53.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d6bbff5f0aada427a1e5a6da5f1f98158182f26556f345ac9e04d36d0ebed650"
 
+[[package]]
+name = "winsafe"
+version = "0.0.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d135d17ab770252ad95e9a872d365cf3090e3be864a34ab46f48555993efc904"
+
 [[package]]
 name = "wit-bindgen"
 version = "0.51.0"