libxl: x86: factor out e820_host_sanitize

libxl: x86: factor out e820_host_sanitize

This function gets the machine E820 map and sanitize it according to PV
guest configuration.

This will be used in later patch. No functional change introduced in
this patch.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@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: Elena Ufimtseva <ufimtseva@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>