Introduce virDomainFSTrim() public API

This will call FITRIM within guest. The API has 4 arguments,
however, only 2 will be used for now (@dom and @minumum).
The rest two are there if in future qemu guest agent learns them.

include/libvirt/libvirt.h.in

@@ -4457,6 +4457,10 @@ int virDomainOpenGraphics(virDomainPtr dom,
 
 int virDomainInjectNMI(virDomainPtr domain, unsigned int flags);
 
+int virDomainFSTrim(virDomainPtr dom,
+                    const char *mountPoint,
+                    unsigned long long minimum,
+                    unsigned int flags);
 
 /**
  * virSchedParameterType: