Include sysemu/hostmem.h less

Include sysemu/hostmem.h less

Move the HostMemoryBackend typedef from sysemu/hostmem.h to
qemu/typedefs.h.  This renders a few inclusions of sysemu/hostmem.h
superfluous; drop them.

Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190812052359.30071-25-armbru@redhat.com>