configure, meson: rename targetos to host_os

This variable is about the host OS, not the target.  It is used a lot
more since the Meson conversion, but the original sin dates back to 2003.
Time to fix it.

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

chardev/meson.build

@@ -12,7 +12,7 @@ chardev_ss.add(files(
   'char-udp.c',
   'char.c',
 ))
-if targetos == 'windows'
+if host_os == 'windows'
   chardev_ss.add(files(
     'char-console.c',
     'char-win-stdio.c',
@@ -23,7 +23,7 @@ else
       'char-fd.c',
       'char-pty.c',
     ), util)
-  if targetos in ['linux', 'gnu/kfreebsd', 'freebsd', 'dragonfly']
+  if host_os in ['linux', 'gnu/kfreebsd', 'freebsd', 'dragonfly']
     chardev_ss.add(files('char-parallel.c'))
   endif
 endif