[PATCH] libxl: use libxl__qemu_qmp_path() instead of open coding

[James Dingwall <james@xxxxxxxxxxxxxx>]

Hi,

I was tracing a problem with the qmp socket and found two open coded
definitions of the path when a helper function is available via
libxl_internal.h which is already included.  The 'const' was needed to
build with the Ubuntu 26.04 tool chain.

Thanks,
James