dump: Rework get_start_block

get_start_block() returns the start address of the first memory block
or -1.

With the GuestPhysBlock iterator conversion we don't need to set the
start address and can therefore remove that code and the "start"
DumpState struct member. The only functionality left is the validation
of the start block so it only makes sense to re-name the function to
validate_start_block()

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Message-Id: <20220811121111.9878-5-frankja@linux.ibm.com>