x86: add MSR API via reg API (#755)

Writing / reading to model specific registers should be as easy as
calling a function, it's a bit stupid to write shell code and run them
just to write/read to a MSR, and even worse, you need more than just a
shellcode to read...

So, add a special register ID called UC_X86_REG_MSR, which should be
passed to uc_reg_write()/uc_reg_read() as the register ID, and then a
data structure which is uc_x86_msr (12 bytes), as the value (always), where:
	Byte	Value		Size
	0	MSR ID		4
	4       MSR val		8

include/unicorn/x86.h

@@ -19,6 +19,13 @@ typedef struct uc_x86_mmr {
     uint32_t flags;     /* not used by GDTR and IDTR */
 } uc_x86_mmr;
 
+// Model-Specific Register structure, use this with UC_X86_REG_MSR (as the register ID) in
+// call to uc_reg_write/uc_reg_read() to manipulate MSRs.
+typedef struct uc_x86_msr {
+    uint32_t rid;
+    uint64_t value;
+} uc_x86_msr;
+
 // Callback function for tracing SYSCALL/SYSENTER (for uc_hook_intr())
 // @user_data: user data passed to tracing APIs.
 typedef void (*uc_cb_insn_syscall_t)(struct uc_struct *uc, void *user_data);
@@ -76,7 +83,7 @@ typedef enum uc_x86_reg {
 	UC_X86_REG_R14D, UC_X86_REG_R15D, UC_X86_REG_R8W, UC_X86_REG_R9W, UC_X86_REG_R10W,
 	UC_X86_REG_R11W, UC_X86_REG_R12W, UC_X86_REG_R13W, UC_X86_REG_R14W, UC_X86_REG_R15W,
 	UC_X86_REG_IDTR, UC_X86_REG_GDTR, UC_X86_REG_LDTR, UC_X86_REG_TR, UC_X86_REG_FPCW,
-	UC_X86_REG_FPTAG,
+	UC_X86_REG_FPTAG, UC_X86_REG_MSR,
 
 	UC_X86_REG_ENDING		// <-- mark the end of the list of registers
 } uc_x86_reg;