qemu: Handle /usr/libexec/qemu-kvm for RHEL systems

[cgwalters]

RHEL-based distributions (including CentOS Stream) use
/usr/libexec/qemu-kvm as the qemu binary path, rather than the
standard qemu-system-{arch}. This causes bcvk to fail when
trying to start VMs on these systems.

Check for the existence of /usr/libexec/qemu-kvm first, and
fall back to the standard qemu-system-{arch} if it doesn't exist.

Fixes: #111
Signed-off-by: Colin Walters walters@verbum.org

[gemini-code-assist]

Code Review

This pull request correctly adds support for RHEL-based systems by checking for /usr/libexec/qemu-kvm as a fallback for the QEMU binary. The logic is sound. I have one suggestion to refactor the implementation slightly to improve readability.

[gemini-code-assist]

While the functional-style chain with map(Ok).unwrap_or_else is correct, it can be a bit dense and harder to read for logic involving fallbacks and potential errors. Refactoring this into an immediately-invoked closure with a more traditional if/else structure makes the sequence of checks clearer and improves overall readability.

Suggested change

	let qemu = std::env::var("QEMU_BIN")
	.ok()
	.map(Ok)
	.unwrap_or_else(|| -> Result<_> {
	// RHEL only supports non-emulated, and qemu is an implementation detail
	// of higher level virt.
	let libexec_qemu = Utf8Path::new("/usr/libexec/qemu-kvm");
	if libexec_qemu.try_exists()? {
	Ok(libexec_qemu.to_string())
	} else {
	let arch = std::env::consts::ARCH;
	Ok(format!("qemu-system-{arch}"))
	}
	})
	.context("Checking for qemu")?;
	let qemu = (|| -> Result<_> {
	if let Some(qemu_bin) = std::env::var("QEMU_BIN").ok() {
	Ok(qemu_bin)
	} else {
	// RHEL only supports non-emulated, and qemu is an implementation detail
	// of higher level virt.
	let libexec_qemu = Utf8Path::new("/usr/libexec/qemu-kvm");
	if libexec_qemu.try_exists()? {
	Ok(libexec_qemu.to_string())
	} else {
	let arch = std::env::consts::ARCH;
	Ok(format!("qemu-system-{arch}"))
	}
	}
	})()
	.context("Checking for qemu")?;

[jmarrero]

lgtm