x86/dpci: move hvm_dpci_isairq_eoi() to generic HVM code

x86/dpci: move hvm_dpci_isairq_eoi() to generic HVM code

The function hvm_dpci_isairq_eoi() has no dependencies on VT-d driver code
and can be moved from xen/drivers/passthrough/vtd/x86/hvm.c to
xen/drivers/passthrough/x86/hvm.c, along with the corresponding copyrights.

Remove the now empty xen/drivers/passthrough/vtd/x86/hvm.c.

Since the function is used only in this file, declare it static.

No functional change intended.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>