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>

tools/meson.build

@@ -1,25 +1,12 @@
-have_virtiofsd = (targetos == 'linux' and
-    have_tools and
-    seccomp.found() and
-    libcap_ng.found() and
-    'CONFIG_VHOST_USER' in config_host)
-
-if get_option('virtiofsd').enabled()
-  if not have_virtiofsd
-    if targetos != 'linux'
-      error('virtiofsd requires Linux')
-    elif not seccomp.found() or not libcap_ng.found()
-      error('virtiofsd requires libcap-ng-devel and seccomp-devel')
-    elif 'CONFIG_VHOST_USER' not in config_host
-      error('virtiofsd needs vhost-user support')
-    else
-      # Disabled all the tools but virtiofsd.
-      have_virtiofsd = true
-    endif
-  endif
-elif get_option('virtiofsd').disabled() or not have_system
-  have_virtiofsd = false
-endif
+have_virtiofsd = get_option('virtiofsd') \
+    .require(targetos == 'linux',
+             error_message: 'virtiofsd requires Linux') \
+    .require(seccomp.found() and libcap_ng.found(),
+             error_message: 'virtiofsd requires libcap-ng-devel and seccomp-devel') \
+    .require('CONFIG_VHOST_USER' in config_host,
+             error_message: 'virtiofsd needs vhost-user-support') \
+    .disable_auto_if(not have_tools and not have_system) \
+    .allowed()
 
 if have_virtiofsd
   subdir('virtiofsd')