fix: preserve mmio state on failed unmap

[Jvlegod]

This PR fix mem_unmap so MMIO callback regions are only updated after uc_mem_unmap succeeds.

For example: if we map a region (0x3000, 0x1000).

Before fix:

before unmap: [(0x3000, 0x1000)]
unmap result: Err(ARG)
after unmap: [(0x3800, 0x800)]

After fix:

before unmap: [(0x3000, 0x1000)]
unmap result: Err(ARG)
after unmap: [(0x3000, 0x1000)]

[wtdcode]

Sorry for late. This looks good to me.