osdep: add qemu_get_local_state_pathname()

This function returns ${prefix}/var/RELATIVE_PATHNAME on POSIX-y systems,
and <CSIDL_COMMON_APPDATA>/RELATIVE_PATHNAME on Win32.

http://msdn.microsoft.com/en-us/library/bb762494.aspx

  [...] This folder is used for application data that is not user
  specific. For example, an application can store a spell-check
  dictionary, a database of clip art, or a log file in the
  CSIDL_COMMON_APPDATA folder. [...]

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