x86/hvm: re-name struct hvm_mmio_handler to hvm_mmio_ops

x86/hvm: re-name struct hvm_mmio_handler to hvm_mmio_ops

The struct just contains three methods and no data, so the name
hvm_mmio_ops more accurately reflects its content. A subsequent patch
introduces a new structure which more accurately warrants the name
hvm_mmio_handler so doing the rename in this purely cosmetic patch avoids
conflating functional and cosmetic changes in a single patch.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>