x86/HVM: fix preemption handling in do_hvm_op() (try 2)

x86/HVM: fix preemption handling in do_hvm_op() (try 2)

Just like previously done for some mem-op hypercalls, undo preemption
using the interface structures (altering it in ways the caller may not
expect) and replace it by storing the continuation point in the high
bits of sub-operation argument.

This also changes the "nr" fields of struct xen_hvm_track_dirty_vram
(operation already limited to 1Gb worth of pages) and struct
xen_hvm_modified_memory to be only 32 bits wide, consistent with those
of struct xen_set_mem{type,access}. If that's not acceptable for some
reason, we'd need to shrink the HVMOP_op_bits (while still enforcing
the [then higher] limit resulting from the need to be able to encode
the continuation).

Whether (and if so how) to adjust xc_hvm_track_dirty_vram(),
xc_hvm_modified_memory(), xc_hvm_set_mem_type(), and
xc_hvm_set_mem_access() to reflect the 32-bit restriction on "nr" is
unclear: If the APIs need to remain stable, all four functions should
probably check that there was no truncation. Preferably their
parameters would be changed to uint32_t or unsigned int, though.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>