xen: is_hvm_{domain,vcpu} should evaluate to false when !CONFIG_HVM

xen: is_hvm_{domain,vcpu} should evaluate to false when !CONFIG_HVM

Turn them into static inline functions which evaluate to false when
CONFIG_HVM is not set. ARM won't be broken because ARM guests are set
to PV type in the hypervisor.

But ARM has plan to switch to HVM guest type inside the hypervisor, so
preemptively introduce CONFIG_HVM for ARM here.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>