qemu: Cargo feature to support `virtfs`

**Is your feature request related to a problem? Please describe.**
I want to fuzz a Linux kernel guest with an injected user-mode executable. I want to inject the executable at runtime, not as part of the kernel build.

**Describe the solution you'd like**
One likely route would be QEMU's `-virtfs` flag. However, this is disabled by LibAFL here:

https://github.com/AFLplusplus/LibAFL/blob/b32557c8ae7e2300657de06382f2761f831c1963/crates/libafl_qemu/libafl_qemu_build/src/build.rs#L210

LibAFL-QEMU already has a Cargo feature for enabling QEMU's `slirp`:

https://github.com/AFLplusplus/LibAFL/blob/b32557c8ae7e2300657de06382f2761f831c1963/crates/libafl_qemu/libafl_qemu_build/Cargo.toml#L28

I would like something similar for `virtfs`. I'm happy to implement if this sounds agreeable.