x86/hvm: only register the r/o subpage ops when needed

x86/hvm: only register the r/o subpage ops when needed

MMIO operation handlers can be expensive to process, hence attempt to
register only those that will be needed by the domain.

Subpage r/o MMIO regions are added exclusively at boot, further limit their
addition to strictly before the initial domain gets created, so by the time
initial domain creation happens Xen knows whether subpage is required or
not.  This allows only registering the MMIO handler when there are
subpage regions to handle.

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