Overzealous limits on MMIO addressing

[pfmooney]

The propolis library defines a pair of limits for sysmem (guest DRAM) and physmem (MMIO accesses):

propolis/lib/propolis/src/vmm/machine.rs

Lines 14 to 16 in e2220ee

	// XXX: Arbitrary limits for now
	pub const MAX_PHYSMEM: usize = 0x80_0000_0000;
	pub const MAX_SYSMEM: usize = 0x40_0000_0000;

The physmem limit in particular poses a challenge for any PCI device which has its 64-bit BAR(s) placed at a physical address which exceeds it. This was found to be at least part of the problem when an illumos guest was unable to attach its nvme driver due to failed accesses to the nvme registers held in a 64-bit BAR placed above the physmem limit.

[luqmana]

Limit was raised in #422 but MMIO mapping should still be made more robust.