I have exactly the same question and have done the same things as you with no success.
I found an entries in my log of
HM: HMR3Init: Falling back to raw-mode: VT-x is not available
VMSetError: F:\tinderbox\win-5.2\src\VBox\VMM\VMMR3\VM.cpp(361) int __cdecl
VMR3Create(unsigned int,const struct VMM2USERMETHODS *,void (__cdecl *)(struct UVM *,void *,int,const char *,unsigned int,const char *,const char *,char *),void *,int (__cdecl *)(struct UVM *,struct VM *,void *),void *,struct VM **,struct UVM **); rc=VERR_SUPDRV_NO_RAW_MODE_HYPER_V_ROOT
00:00:05.088846
VMSetError: Raw-mode is unavailable courtesy of Hyper-V. 00:00:05.089946
ERROR [COM]: aRC=E_FAIL (0x80004005) aIID={872da645-4a9b-1727-bee2-5585105b9eed} aComponent={ConsoleWrap} aText={Raw-mode is unavailable courtesy of Hyper-V. (VERR_SUPDRV_NO_RAW_MODE_HYPER_V_ROOT)}, preserve=false aResultDetail=0 00:00:05.090271 Console: Machine state changed to 'PoweredOff'
My chip says it has VT-x and is on in the Bios but the log says not
HM: HMR3Init: Falling back to raw-mode: VT-x is not available
I have a 6 month old Lenovo Yoga with 2.7-GHz Intel Core i7-7500U
I have tried the following, but it didn't work for me.
From this thread https://forums.virtualbox.org/viewtopic.php?t=77120#p383348 I tried disabling Device Guard but Windows wouldn't shut down so I reenabled it.
I used this path ....
On the host operating system, click Start > Run, type gpedit.msc, and click Ok. The Local group Policy Editor opens.
Go to Local Computer Policy > Computer Configuration > Administrative Templates > System > Device Guard > Turn on Virtualization Based Security. Select Disabled.