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>

hypervisor/src/kvm/aarch64/mod.rs

@@ -82,6 +82,7 @@ macro_rules! arm64_core_reg_id {
 }
 
 /// Specifies whether a particular register is a system register or not.
+///
 /// The kernel splits the registers on aarch64 in core registers and system registers.
 /// So, below we get the system registers by checking that they are not core registers.
 ///