libxc: do some retries in xc_cpupool_removecpu() for EBUSY case

libxc: do some retries in xc_cpupool_removecpu() for EBUSY case

The hypervisor might return EBUSY when trying to remove a cpu from a
cpupool when a domain running in this cpupool has pinned a vcpu
temporarily. Do some retries in this case, perhaps the situation
cleans up.

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>