qga: add QGA_CONF environment variable

Having a environment variable allows to override default configuration
path, useful for testing. Note that this can't easily be an argument,
since loading config is done before parsing the arguments.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
