qga: Support Unicode paths in guest-file-open on win32

Currently, the win32 port of QEMU Guest Agent does not properly handle Unicode
paths. The JSON decoder produces a valid UTF-8 path string, but this is passed
directly to CreateFileA, which is expecting an ANSI string and not UTF-8. This
leads to mangled filenames.

This patch follows the example of qmp_guest_set_user_password() and uses
g_utf8_to_utf16() to convert the string to UTF-16 and calls CreateFileW()
explicitly.

Signed-off-by: Jonathon Reinhart <jreinhart@cc-sw.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>