docs: Fix virtiofsd.1 location

Patch 6a7e2bbee5 docs: add virtiofsd(1) man page introduced new man
page virtiofsd.1. Unfortunately, wrong file location is used as
source for install command. This cause installation of docs fail.

Fixing wrong location so installation is successful.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
Message-Id: <1608747495.7195569.1581513386780.JavaMail.zimbra@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Makefile

@@ -865,7 +865,7 @@ ifdef CONFIG_VIRTFS
 	$(INSTALL_DATA) $(MANUAL_BUILDDIR)/interop/virtfs-proxy-helper.1 "$(DESTDIR)$(mandir)/man1"
 endif
 ifeq ($(CONFIG_LINUX)$(CONFIG_SECCOMP)$(CONFIG_LIBCAP_NG),yyy)
-	$(INSTALL_DATA) docs/interop/virtiofsd.1 "$(DESTDIR)$(mandir)/man1"
+	$(INSTALL_DATA) $(MANUAL_BUILDDIR)/interop/virtiofsd.1 "$(DESTDIR)$(mandir)/man1"
 endif
 
 install-datadir: