qga/commands-posix: Fix bug in guest-fstrim

The FITRIM ioctl updates the fstrim_range structure it receives. This
way the caller can determine how many bytes were trimmed. The
guest-fstrim logic reuses the same fstrim_range for each filesystem,
effectively limiting each filesystem to trim at most as much as the
previous was able to trim.

If a previous filesystem would have trimmed 0 bytes, than the next
filesystem would report an error 'Invalid argument' because a FITRIM
request with length 0 is not valid.

This change resets the fstrim_range structure for each filesystem.

Signed-off-by: Justin Ossevoort <justin@quarantainenet.nl>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
