xen: move common variables off of sys/x86/xen/hvm.c

The xen_domain_type and HYPERVISOR_shared_info variables are shared by
all Xen architectures, so they should be in common rather than
reimplemented by each architecture.

hvm_start_flags is used by xen_initial_domain() and so needs to be in
common.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D28982