libxl: don't needlessly report "highmem" in use

libxl: don't needlessly report "highmem" in use

Due to the unconditional updating of dom->highmem_end in
libxl__domain_device_construct_rdm() I've observed on a 2Gb HVM guest
with a passed through device (without overly large BARs, and with no RDM
ranges at all)

(d2) RAM in high memory; setting high_mem resource base to 100000000
...
(d2) E820 table:
(d2)  [00]: 00000000:00000000 - 00000000:000a0000: RAM
(d2)  HOLE: 00000000:000a0000 - 00000000:000d0000
(d2)  [01]: 00000000:000d0000 - 00000000:00100000: RESERVED
(d2)  [02]: 00000000:00100000 - 00000000:7f800000: RAM
(d2)  HOLE: 00000000:7f800000 - 00000000:fc000000
(d2)  [03]: 00000000:fc000000 - 00000001:00000000: RESERVED
(d2)  [04]: 00000001:00000000 - 00000001:00000000: RAM

both of which aren't really appropriate in this case. Arrange for this
to not happen.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>