tools/libxl: parse optional start gfn from the iomem config option

tools/libxl: parse optional start gfn from the iomem config option

Currently, the "iomem" domU config option allows to specify a machine
address range to be mapped to the domU. However, there is no way to
specify the guest address range used for the mapping. This commit
extends the iomem option handling code to parse an additional, optional
parameter: this parameter, if given, specifies the start guest address
used for the mapping; if no start guest address is given, a 1:1 mapping
is performed as default.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Acked-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@citrix.com>
Cc: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Paolo Valente <paolo.valente@unimore.it>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Eric Trudeau <etrudeau@broadcom.com>
Cc: Viktor Kleinik <viktor.kleinik@globallogic.com>
Cc: Andrii Tseglytskyi <andrii.tseglytskyi@globallogic.com>