x86/hvm/ioreq: allow ioreq servers to use HVM_PARAM_[BUF]IOREQ_PFN

x86/hvm/ioreq: allow ioreq servers to use HVM_PARAM_[BUF]IOREQ_PFN

Since commit 2c257bd6 "x86/hvm: remove default ioreq server (again)" the
GFNs allocated by the toolstack and set in HVM_PARAM_IOREQ_PFN and
HVM_PARAM_BUFIOREQ_PFN have been unused. This patch allows them to be used
by (non-default) ioreq servers.

While in the area, also make sure HVM_PARAM_[BUF]IOREQ_PFN can only be set
once. These parameters should have always been in the 'set once' category
but this has, so far, not been enforced.

NOTE: This fixes a compatibility issue. A guest created on a version of
      Xen that pre-dates the initial ioreq server implementation and then
      migrated in will currently fail to resume because its migration
      stream will lack values for HVM_PARAM_IOREQ_SERVER_PFN and
      HVM_PARAM_NR_IOREQ_SERVER_PAGES *unless* the system has an
      emulator domain that uses direct resource mapping (which depends
      on the version of privcmd it happens to have) in which case it
      will not require use of GFNs for the ioreq server shared
      pages.

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