x86/hvm: take a reference on ioreq server emulating domain

x86/hvm: take a reference on ioreq server emulating domain

When an ioreq server is created the code currently stores the id
of the emulating domain, but does not take a reference on that domain.

This patch modifies the code to hold a reference for the lifetime of the
ioreq server.

NOTE: ioreq servers are either destroyed explicitly or destroyed implicitly
      in context of XEN_DOMCTL_destroydomain.
      If the emulating domain is shut down prior to the target then the
      any domain reference held by an ioreq server will prevent it from
      being destroyed. However, if an emulating domain is shut down prior
      to its target then it is likely that the target's vcpus will block
      fairly quickly waiting for emulation that will never occur, and when
      the target domain is destroyed the reference on the zombie emulating
      domain will be dropped allowing both to be cleaned up.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>