x86/paravirt: Fix build PARAVIRT_XXL=y without XEN_PV

Kernel fails to compile with PARAVIRT_XXL=y if XEN_PV is not enabled:

	ld.lld: error: undefined symbol: xen_iret

It happens because INTERRUPT_RETURN defined to use xen_iret if
CONFIG_PARAVIRT_XXL enabled regardless of CONFIG_XEN_PV.

The issue is not visible in the current kernel because CONFIG_XEN_PV is
the only user of CONFIG_PARAVIRT_XXL and there's no way to enable them
separately.

Rework code to define INTERRUPT_RETURN based on CONFIG_XEN_PV, not
CONFIG_PARAVIRT_XXL.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lkml.kernel.org/r/20211130185533.32658-1-kirill.shutemov@linux.intel.com