hv: add time modification of vrtc

VRTC for hv used to ignore writes to vRTC register.

This patch add time modification to vRTC.
Add base RTC time and TSC offset to get the current time. Convert
current time to vRTC register values (`struct rtcdev`). Then
modify a register, and calculate a new time. Get RTC offset by
substrcting new time from current time.
Thereafter, add RTC offset also when get current time. Then user
can get the modified time.

Tracked-On: #7440
Signed-off-by: Yuanyuan Zhao <yuanyuan.zhao@linux.intel.com>
Reviewed-by: Junjie Mao <junjie.mao@intel.com>

hypervisor/include/dm/vrtc.h

@@ -34,6 +34,8 @@ struct acrn_vrtc {
 	uint32_t	addr;           /* RTC register to read or write */
 
 	time_t		base_rtctime;	/* Base time calulated from physical rtc register. */
+	time_t		offset_rtctime;	/* RTC offset against base time. */
+
 	uint64_t	base_tsc;	/* Base tsc value */
 
 	struct rtcdev	rtcdev;		/* RTC register */