hw/pci-host/q35.c: Factor out creation of SMRAM MRs

mch_realize has a large section that deals with initializing the
SMRAM-specific MemoryRegions.  Currently we do an early return from
the realize function if mch->has_smm_ranges is false, but this has
the potential for bugs if somebody adds new code at the end of the
function that isn't SMM-specific.  Pull the MR init code out into its
own function, so we can do the smm-ranges specific handling in the
realize function in a more obvious way.

This commit shouldn't change behaviour at all.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260708121011.1653365-3-peter.maydell@linaro.org>