meson: use .require() and .disable_auto_if() method for features
The method is now in 0.59, using it simplifies some conditionals.
There is a small change, which is to build virtfs-proxy-helper in a
tools-only build. This is done for consistency with other tools,
which are not culled by the absence of system emulator binaries.
.disable_auto_if() would also be useful to check for packages,
for example