x86/xen: ignore error to fetch memory map in xen_arch_init_physmem()

Some implementations of Xen don't expose the XENMEM_memory_map hypercall.
Shallow the error from XENMEM_memory_map in xen_arch_init_physmem() and just
return 0. This will fallback to using the non-arch specific mechanism for
allocating scratch space.

Reported by:	cperciva
Reviewed by:	Elliott Mitchell
Fixes:	69c4748 ('x86/xen: use UNUSABLE e820 regions for external mappings')
Sponsored by:	Cloud Software Group
Differential revision:	https://reviews.freebsd.org/D46205

sys/x86/xen/hvm.c

@@ -563,7 +563,7 @@ xen_arch_init_physmem(device_t dev, struct rman *mem)
 	set_xen_guest_handle(memmap.buffer, smap);
 	error = HYPERVISOR_memory_op(XENMEM_memory_map, &memmap);
 	if (error != 0)
-		return (error);
+		return (0);
 
 	/*
 	 * Fill with UNUSABLE regions, as it's always fine to use those for