Add support for --reflink to the virsh 'vol-create-from' and 'vol-clone'
commands to signify usage of the VIR_STORAGE_VOL_CREATE_REFLINK flag in the
ensuing virStorageVolCreateXMLFrom API call.

Updated the man page to describe the new flag.
Signed-off-by: NChen Hanxiao <chenhanxiao@cn.fujitsu.com>