x86/hvm: Disallow moving the APIC MMIO window

x86/hvm: Disallow moving the APIC MMIO window

See the code comment for a full discussion, but in short: guests which
currently run under Xen don't move the window, because moving it has never
worked properly.  Implementing support for moving the window is never going to
work architecturally unless we switch to per-vcpu P2Ms (which seems very
unlikely), and would still be a substantial quantity of work for a feature
which is unused in practice.

Take the opportunity to rename vlapic_msr_set() to be consistent with the
other MSR handling functions, and return X86EMUL_* constants.  Add logic to
check for reserved bits, including refusing x2APIC mode if it has not been
offered to the guest.  Move the guest_{rd,wr}msr_x2apic() declarations into
vlapic.h which is a more appropriate place for them to live.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>