qga: determine default state dir and pidfile dynamically

qga: determine default state dir and pidfile dynamically

No effective change on POSIX, but on Win32 the defaults come from the
environment / session.

Since commit 39097daf ("qemu-ga: use key-value store to avoid recycling fd
handles after restart") we've relied on the state directory for the fd
handles' key-value store. Even though we don't support the guest-file-*
commands on win32 yet, the key-value store is written, and it's the first
use of the state directory on win32. We should have a sensible default for
its location.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>