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>

contrib/ivshmem-client/meson.build

@@ -1,4 +1,4 @@
 executable('ivshmem-client', files('ivshmem-client.c', 'main.c'), genh,
            dependencies: glib,
-           build_by_default: targetos == 'linux',
+           build_by_default: host_os == 'linux',
            install: false)

contrib/ivshmem-server/meson.build

@@ -1,4 +1,4 @@
 executable('ivshmem-server', files('ivshmem-server.c', 'main.c'), genh,
            dependencies: [qemuutil, rt],
-           build_by_default: targetos == 'linux',
+           build_by_default: host_os == 'linux',
            install: false)

contrib/vhost-user-blk/meson.build

@@ -1,4 +1,4 @@
 executable('vhost-user-blk', files('vhost-user-blk.c'),
            dependencies: [qemuutil, vhost_user],
-           build_by_default: targetos == 'linux',
+           build_by_default: host_os == 'linux',
            install: false)

contrib/vhost-user-input/meson.build

@@ -1,4 +1,4 @@
 executable('vhost-user-input', files('main.c'),
            dependencies: [qemuutil, vhost_user],
-           build_by_default: targetos == 'linux',
+           build_by_default: host_os == 'linux',
            install: false)

contrib/vhost-user-scsi/meson.build

@@ -1,6 +1,6 @@
 if libiscsi.found()
   executable('vhost-user-scsi', files('vhost-user-scsi.c'),
              dependencies: [qemuutil, libiscsi, vhost_user],
-             build_by_default: targetos == 'linux',
+             build_by_default: host_os == 'linux',
              install: false)
 endif