x86/HVM: RTC code must be in line with WAET flags passed by hvmloader

x86/HVM: RTC code must be in line with WAET flags passed by hvmloader

With hvmloader telling the guest that it may skip REG_C reads during
the processing of RTC interrupts, the emulation code must not depend
upon these reads to occur. Introduce two modes of operation for the
emulation code, and short of a HVM parameter (too late to be
introduced for 4.3) hard code the mode determination to always assume
that Windows-conforming one for the time being.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Roger Pau Monné <roger.pau@citrix.com> (FreeBSD guest)