misc: Fix various warnings from clippy 0.1.82

An example warning output is:

error: first doc comment paragraph is too long
   --> virtio-devices/src/lib.rs:158:1
    |
158 | / /// Convert an absolute address into an address space (GuestMemory)
159 | | /// to a host pointer and verify that the provided size define a valid
160 | | /// range within a single memory region.
161 | | /// Return None if it is out of bounds or if addr+size overlaps a single region.
    | |_
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#too_long_first_doc_paragraph
    = note: `-D clippy::too-long-first-doc-paragraph` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::too_long_first_doc_paragraph)]`

Signed-off-by: Bo Chen <chen.bo@intel.com>

arch/src/x86_64/mod.rs

@@ -905,6 +905,7 @@ pub fn configure_vcpu(
 }
 
 /// Returns a Vec of the valid memory addresses.
+///
 /// These should be used to configure the GuestMemory structure for the platform.
 /// For x86_64 all addresses are valid from the start of the kernel except a
 /// carve out at the end of 32bit address space.