qga: guest exec functionality

qga: guest exec functionality

Guest-exec rewritten in platform-independent style with glib spawn.

Child process is spawn asynchronously and exit status can later
be picked up by guest-exec-status command.

stdin/stdout/stderr of the child now is redirected to /dev/null
Later we will add ability to specify stdin in guest-exec command
and to get collected stdout/stderr with guest-exec-status.

Signed-off-by: Yuri Pudgorodskiy <[email protected]>
Signed-off-by: Denis V. Lunev <[email protected]>
CC: Michael Roth <[email protected]>
* use g_new0 in place of g_malloc for GuestExec struct
* commit msg spelling fixes
* s/inp-data/input-data
* document capture-input mode as false by default
* use GetProcessId() for pids on w32 instead of casting HANDLE
Signed-off-by: Michael Roth <[email protected]>