Restructure coverage (#569)

Change the structure of coverage for more extendability
Move coverage functions into riscv_instr and change most of functions to
virtual
Commented out most of floating point related functions for now, will move
them to riscv_floating_point_instr in another PR

Author: weicaiyang

src/isa/riscv_instr.sv

@@ -649,4 +649,6 @@ class riscv_instr extends uvm_object;
     imm_str = $sformatf("%0d", $signed(imm));
   endfunction
 
+  `include "isa/riscv_instr_cov.svh"
+
 endclass