virsh: Create macro for common "file" option

virsh: Create macro for common "file" option

Rather than continually cut-n-paste the strings into each command,
create a common macro to be used generically. The macro will take a
single argument _helpstr for the less common help string for each
command option.  Note that only file options using "OT_DATA" and
"OFLAG_REQ" will be replace - others are left as is.

Signed-off-by: John Ferlan <jferlan@redhat.com>