x86/HVM: fold hvm_io_assist() into its only caller

x86/HVM: fold hvm_io_assist() into its only caller

While there are two call sites, the function they're in can be slightly
re-arranged such that the code sequence can be added at its bottom. Note
that the function's only caller has already checked sv->pending, and
that the prior while() loop was just a slightly more fancy if()
(allowing an early break out of the construct).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>