qemu: Cargo feature to support `virtfs`

[langston-barrett]

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:

LibAFL/crates/libafl_qemu/libafl_qemu_build/src/build.rs

Line 210 in b32557c

.arg("--disable-virtfs")

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

LibAFL/crates/libafl_qemu/libafl_qemu_build/Cargo.toml

Line 28 in b32557c

slirp = [] # build qemu with host libslirp (for user networking)

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