xen/ioreq: Remove "hvm" prefixes from involved function names

xen/ioreq: Remove "hvm" prefixes from involved function names

This patch removes "hvm" prefixes and infixes from IOREQ related
function names in the common code and performs a renaming where
appropriate according to the more consistent new naming scheme:
- IOREQ server functions should start with "ioreq_server_"
- IOREQ functions should start with "ioreq_"

A few function names are clarified to better fit into their purposes:
handle_hvm_io_completion -> vcpu_ioreq_handle_completion
hvm_io_pending           -> vcpu_ioreq_pending
hvm_ioreq_init           -> ioreq_domain_init
hvm_alloc_ioreq_mfn      -> ioreq_server_alloc_mfn
hvm_free_ioreq_mfn       -> ioreq_server_free_mfn

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
CC: Julien Grall <julien.grall@arm.com>
[On Arm only]
Tested-by: Wei Chen <Wei.Chen@arm.com>