feat(qemu): Feature for enabling virtfs

[langston-barrett]

Description

Fixes #3824.

Checklist

• I have run ./scripts/precommit.sh and addressed all comments

^ this failed for me with a message about openssl

[domenukk]

error: features "usermode" and "systemmode" cannot be used together
  --> crates/libafl_qemu/libafl_qemu_sys/build_linux.rs:11:13
   |
11 |             compile_error!(concat!("features \"", $first, "\" and \"", $rest, "\" cannot be used together"));
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
27 |     assert_unique_feature!("usermode", "systemmode");
   |     ------------------------------------------------ in this macro invocation
   |
   = note: this error originates in the macro `assert_unique_feature

probably happens because some --all-features thing enables usermode while you enable systemmode here...

@rmalmain maybe it would be a good idea to have usermode as default and systemmode as feature - that way we have one less exclusive feature flag?