Provide separate helpers for guest enter vtime accounting (in addition to
the existing guest exit helpers), and move all vtime accounting helpers
to vtime.h where the existing #ifdef infrastructure can be leveraged to
better delineate the different types of accounting.  This will also allow
future cleanups via deduplication of context tracking code.

Opportunstically delete the vtime_account_kernel() stub now that all
callers are wrapped with CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y.

No functional change intended.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210505002735.1684165-6-seanjc@google.com