KVM: selftests: Stash backing_src_type in struct userspace_mem_region

Add the backing_src_type into struct userspace_mem_region. This struct
already stores a lot of info about memory regions, except the backing
source type.  This info will be used by a future commit in order to
determine the method for punching a hole.
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221017195834.2295901-7-ricarkol@google.com

tools/testing/selftests/kvm/include/kvm_util_base.h

@@ -34,6 +34,7 @@ struct userspace_mem_region {
 	struct sparsebit *unused_phy_pages;
 	int fd;
 	off_t offset;
+	enum vm_mem_backing_src_type backing_src_type;
 	void *host_mem;
 	void *host_alias;
 	void *mmap_start;

tools/testing/selftests/kvm/lib/kvm_util.c

@@ -929,6 +929,7 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
 			    vm_mem_backing_src_alias(src_type)->name);
 	}
 
+	region->backing_src_type = src_type;
 	region->unused_phy_pages = sparsebit_alloc();
 	sparsebit_set_num(region->unused_phy_pages,
 		guest_paddr >> vm->page_shift, npages);