HV: Modularize vtd.c to remove acrn_vm usage

This patch is a modified one. It removes the usage
of acrn_vm struct from inside vtd.c.
It also puts struct iommu_domain inside vtd.h,
from vtd.c.
It modifies the signature of init_iommu_domain
in order to remove dependency on acrn_vm from
inside vtd.c.
Incorporated comments from Jason and Eddie.
Changed the name of sos_vm_domain to
fallback_iommu_domain
Removed any reference of sos_vm from vtd.[c|h]
files, including comments.

Tracked-On: #2496
Signed-off-by: Arindam Roy <arindam.roy@intel.com>
Reviewed-by: Jason Chen CJ <jason.cj.chen@intel.com>

hypervisor/common/hypercall.c

@@ -9,6 +9,7 @@
 #include <hypercall.h>
 #include <version.h>
 #include <reloc.h>
+#include <vtd.h>
 
 #define ACRN_DBG_HYCALL	6U