Fix `setvar` error check on x86-64

[Ivan-Velickovic]

microkit/tool/microkit/src/sdf.rs

Lines 982 to 988 in 40a2c91

	Arch::X86_64 => {
	return Err(value_error(
	xml_sdf,
	node,
	"setvar with 'region_paddr' for MR without a specified paddr is unsupported on x86_64".to_string(),
	));
	}

This error check is necessary since on x86-64 we don't know the physical address of regions that are implicity allocated until run-time so don't do setvar, but it is incorrect right now since it doesn't actually check if region_paddr points to a MR that has phys_addr specified.