libxl: introduce libxl__vnuma_config_check

libxl: introduce libxl__vnuma_config_check

This function is used to check whether vNUMA configuration (be it
auto-generated or supplied by user) is valid.

Define a new error code ERROR_VNUMA_CONFIG_INVALID.

The checks performed can be found in the comment of the function.

This vNUMA function (and future ones) is placed in a new file called
libxl_vnuma.c

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Elena Ufimtseva <ufimtseva@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>