gnu: qemu-kvm: Add patch to have multiple SMB shares.

* gnu/packages/qemu.scm (qemu-kvm/smb-shares): New variable.
* gnu/packages/patches/qemu-multiple-smb-shares.patch: New file.
* Makefile.am (dist_patch_DATA): Add it.

Makefile.am

@@ -202,6 +202,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/mcron-install.patch			\
   gnu/packages/patches/perl-no-sys-dirs.patch			\
   gnu/packages/patches/procps-make-3.82.patch			\
+  gnu/packages/patches/qemu-multiple-smb-shares.patch		\
   gnu/packages/patches/readline-link-ncurses.patch		\
   gnu/packages/patches/shishi-gets-undeclared.patch		\
   gnu/packages/patches/tar-gets-undeclared.patch		\

gnu/packages/patches/qemu-multiple-smb-shares.patch

@@ -0,0 +1,20 @@
+This file extends `-smb' to add a share for the Nix store, and changes
+the name of the default share.
+
+--- a/net/slirp.c
++++ b/net/slirp.c
+@@ -515,8 +515,12 @@ static int slirp_smb(SlirpState* s, const char *exported_dir,
+             "log file=%s/log.smbd\n"
+             "smb passwd file=%s/smbpasswd\n"
+             "security = share\n"
+-            "[qemu]\n"
+-            "path=%s\n"
++            "[store]\n"
++            "path=/nix/store\n"
++            "read only=yes\n"
++            "guest ok=yes\n"
++            "[xchg]\n"
++            "path=%s/xchg\n"
+             "read only=no\n"
+             "guest ok=yes\n",
+             s->smb_dir,

gnu/packages/qemu.scm

@@ -22,6 +22,7 @@
   #:use-module (guix utils)
   #:use-module ((guix licenses) #:select (gpl2))
   #:use-module (guix build-system gnu)
+  #:use-module (gnu packages)
   #:use-module (gnu packages autotools)
   #:use-module (gnu packages pkg-config)
   #:use-module (gnu packages glib)
@@ -96,6 +97,18 @@ underway to get the required changes upstream.")
     ;; Many files are GPLv2+, but some are GPLv2-only---e.g., `memory.c'.
     (license gpl2)))
 
+(define-public qemu-kvm/smb-shares
+  ;; A patched QEMU-KVM where `-net smb' yields two shares instead of one: one
+  ;; for the store, and another one for exchanges with the host.
+  (package (inherit qemu-kvm)
+    (name "qemu-kvm-with-multiple-smb-shares")
+    (inputs `(,@(package-inputs qemu-kvm)
+              ("patch/smb-shares"
+               ,(search-patch "qemu-multiple-smb-shares.patch"))))
+    (arguments
+     `(#:patches (list (assoc-ref %build-inputs "patch/smb-shares"))
+       ,@(package-arguments qemu-kvm)))))
+
 (define-public qemu
   ;; The real one, with a complete target list.
   (package (inherit qemu-kvm)