EmbeddedPkg/VirtualRealTimeClockLib: Fix correctness issues

EmbeddedPkg/VirtualRealTimeClockLib: Fix correctness issues

LibGetTime():
- Two variables were used for the epoch, where only one should have been [*].
- Also harmonize variable name to match the one used in LibSetTime.
LibSetTime():
- Address possible underflows if time is set to start of epoch.
- Ensure that time being read does actually match time that was manually
  set (plus the time elapsed since), by subtracting number of seconds
  since reset.

[*] This fixes a build breakage, since one of these variables was set but
    never used, triggering a compiler diagnostic at some optimization levels.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>