x86/vHPET: use rwlock instead of simple one

x86/vHPET: use rwlock instead of simple one

This namely benefits guests heavily reading the main counter, but not
touching the HPET much otherwise. Note that due to the way
hpet_get_comparator() works hpet_read() has to special cases reads from
the comparator registers and use a write lock there instead of the read
one used for all other registers.

Signed-off-by: Jan Beulich <jbeulich@suse.com>