xen: change quality of the MADT ACPI enumerator

xen: change quality of the MADT ACPI enumerator

Lower the quality of the MADT ACPI enumerator, so on Xen Dom0 we can
force the usage of the Xen mptable enumerator even when ACPI is
detected.

This is needed because Xen might restrict the number of vCPUs
available to Dom0, but the MADT ACPI table parsed in FreeBSD is the
native one (which enumerates all the CPUs available in the system).

Sponsored by: Citrix Systems R&D
Reviewed by: gibbs

x86/acpica/madt.c:
 - Lower MADT enumerator quality to -50.

x86/xen/pvcpu_enum.c:
 - Rise Xen PV enumerator to 0.