qga: guest-exec simple stdin/stdout/stderr redirection

Implemented with base64-encoded strings in qga json protocol.
Glib portable GIOChannel is used for data I/O.

Optinal stdin parameter of guest-exec command is now used as
stdin content for spawned subprocess.

If capture-output bool flag is specified, guest-exec redirects out/err
file descriptiors internally to pipes and collects subprocess
output.

Guest-exe-status is modified to return this collected data to requestor
in base64 encoding.

Signed-off-by: Yuri Pudgorodskiy <yur@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
* switch from 'struct GuestIOExecData' to 'GuestIOExecData'
* s/TRUE/true/g, s/FALSE/false/g for gboolean return values
* s/inp_data/input_data/
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>