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: 69c47485b5f9 ('x86/xen: use UNUSABLE e820 regions for external mappings')
Sponsored by: Cloud Software Group
Differential revision: https://reviews.freebsd.org/D46205