qemu-options: avoid #if in spicevmc texi help

Preprocessor directives cannot be used in STEXI/ETEXI sections since
they are not passed through the preprocessor.  The spicevmc chardev
option help currently uses #if, which is included verbatim in the man
page output.

Fix this by simply stating that spicevmc chardevs are available only in
builds with spice support.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

qemu-options.hx

@@ -1673,15 +1673,15 @@ Connect to a local parallel port.
 @option{path} specifies the path to the parallel port device. @option{path} is
 required.
 
-#if defined(CONFIG_SPICE)
 @item -chardev spicevmc ,id=@var{id} ,debug=@var{debug}, name=@var{name}
 
+@option{spicevmc} is only available when spice support is built in.
+
 @option{debug} debug level for spicevmc
 
 @option{name} name of spice channel to connect to
 
 Connect to a spice virtual machine channel, such as vdiport.
-#endif
 
 @end table
 ETEXI