qga/qmp_guest_fstrim: Return per path fstrim result

qga/qmp_guest_fstrim: Return per path fstrim result

The current guest-fstrim support only returns an error if some
mountpoint was unable to be trimmed, skipping any possible additional
mountpoints. The result of the TRIM operation itself is also discarded.

This change returns a per mountpoint result of the TRIM operation. If an
error occurs on some mountpoints that error is returned and the
guest-fstrim continue with any additional mountpoints.

The returned values for errors, minimum and trimmed are dependant on the
filesystem, storage stacks and kernel version.

Signed-off-by: Justin Ossevoort <justin@quarantainenet.nl>
* s/type/struct/ in schema type definitions
* moved version annotation for new guest-fstrim return field to
  the field itself rather than applying to the entire command
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>