arm: clobber only argument registers

arm: clobber only argument registers

Previously it was declared that r1..r4 would all be clobbered by all
hypercalls. Instead declare that only actually used hypercall argument
registers are clobbered. This is more inline with generally expected
conventions and allows for more optimal code in the caller in some cases.

This is an ABI change, although an older guest which expects more things to be
clobbered than we do now won't be adversely impacted.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>