qga: removing switch statements, adding run_process_child

This is a cleanup of the resulting code after detaching
pmutils and Linux sys state file logic:

- remove the SUSPEND_MODE_* macros and use an enumeration
instead. At the same time, drop the switch statements
at the start of each function and use the enumeration
index to get the right binary/argument;

- create a new function called run_process_child(). This
function uses g_spawn_sync() to execute a shell command,
returning the exit code. This is a common operation in the
pmutils functions and will be used in the systemd implementation
as well, so this function will avoid code repetition.

There are more places inside commands-posix.c where this new
run_process_child function can also be used, but one step
at a time.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
*check/propagate local_err before setting errp directly
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>