qga: implement file commands for Windows guest

qga: implement file commands for Windows guest

The following commands are implemented:
- guest_file_open
- guest_file_close
- guest_file_write
- guest_file_read
- guest_file_seek
- guest_file_flush

Motivation is quite simple: Windows guests should be supported with the
same set of features as Linux one. Also this patch is a prerequisite for
Windows guest-exec command support.

Signed-off-by: Olga Krishtal <[email protected]>
Signed-off-by: Denis V. Lunev <[email protected]>
CC: Michael Roth <[email protected]>
Signed-off-by: Michael Roth <[email protected]>