qemu: support disk filenames with comma

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>