meson: use .require() and .disable_auto_if() method for features

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

-linux_io_uring = not_found
-if not get_option('linux_io_uring').auto() or have_block
-  linux_io_uring = dependency('liburing', required: get_option('linux_io_uring'),
-                              method: 'pkg-config', kwargs: static_kwargs)
-endif
+linux_io_uring = dependency('liburing',
+  required: get_option('linux_io_uring').disable_auto_if(not have_block),
+  method: 'pkg-config', kwargs: static_kwargs)

This change however is much larger and I am not sure about the improved
readability, so I am not performing it right now.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>