tools: libxl: hide selection of hvmloader by default.

tools: libxl: hide selection of hvmloader by default.

This should never have been exposed to users as something they are
required to think about, unless they want to.

At the libxl API level:
      * Move libxl_domain_build_info.kernel into the PV side of the
        tagged union (using this field to specify both PV kernel and
        hvmloader is confusing)
      * Add hvmloader (a string) to the HVM side of the tagged union.
        This defaults to NULL and libxl will DTRT with that default
        but still allow libxl users to specify something explicit if
        they want.

At the xl level:
      * WARN if an HVM guest cfg uses the "kernel" config option, and
        direct users to the "hvmloader_override" option if they really
        do not want the default hvmloader.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>