i386/pc: factor out device_memory base/size to helper

i386/pc: factor out device_memory base/size to helper

Move obtaining hole64_start from device_memory memory region base/size
into an helper alongside correspondent getters in pc_memory_init() when
the hotplug range is unitialized. While doing that remove the memory
region based logic from this newly added helper.

This is the final step that allows pc_pci_hole64_start() to be callable
at the beginning of pc_memory_init() before any memory regions are
initialized.

Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20220719170014.27028-9-joao.m.martins@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>