timers: use INT64_MAX as max expiration

timers: use INT64_MAX as max expiration

Currently, the max expiration time is 2147483647ns(INT32_MAX ns). This
is enough when guest is busy, but when guest is idle, the next timer
will be later than INT32_MAX ns. And those meaningless alarm will harm
the pkg C-state.

PS: Since the overflow will not happen with the expression((delta /
1000) + (delta % 1000 > 0 ? 1 : 0)), so i also removed the comments"
To avoid problems with overflow limit this to 2^32."

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>