qemu: support disk filenames with comma

If there is a disk file with a comma in the name, QEmu expects a double
comma instead of a single one (e.g., the file "virtual,disk.img" needs
to be specified as "virtual,,disk.img" in QEmu's command line). This
patch fixes libvirt to work with that feature. Fix RHBZ #801036.

Based on an initial patch by Crístian Viana.

* src/util/buf.h (virBufferEscape): Alter signature.
* src/util/buf.c (virBufferEscape): Add parameter.
(virBufferEscapeSexpr): Fix caller.
* src/qemu/qemu_command.c (qemuBuildRBDString): Likewise.  Also
escape commas in file names.
(qemuBuildDriveStr): Escape commas in file names.
* docs/schemas/basictypes.rng (absFilePath): Relax RNG to allow
commas in input file names.
* tests/qemuxml2argvdata/*-disk-drive-network-sheepdog.*: Update
test.

Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qemuxml2argvdata/qemuxml2argv-disk-drive-network-sheepdog.xml

@@ -15,13 +15,13 @@
   <devices>
     <emulator>/usr/bin/qemu</emulator>
     <disk type='block' device='disk'>
-      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <source dev='/dev/HostVG/QEMU,Guest,,1'/>
       <target dev='hda' bus='ide'/>
       <address type='drive' controller='0' bus='0' target='0' unit='0'/>
     </disk>
     <disk type='network' device='disk'>
       <driver name='qemu' type='raw'/>
-      <source protocol='sheepdog' name='image'>
+      <source protocol='sheepdog' name='image,with,commas'>
         <host name='example.org' port='6000'/>
       </source>
       <target dev='vda' bus='virtio'/>