x86/altp2m: Rework #VE enable/disable paths

x86/altp2m: Rework #VE enable/disable paths

Split altp2m_vcpu_{enable,disable}_ve() out of the
HVMOP_altp2m_vcpu_{enable,disable}_notify marshalling logic.  A future change
is going to need to call altp2m_vcpu_disable_ve() from the domain_kill() path.

While at it, clean up the logic in altp2m_vcpu_{initialise,destroy}().
altp2m_vcpu_reset() has no external callers, so fold it into its two
callsites.  This in turn allows for altp2m_vcpu_destroy() to reuse
altp2m_vcpu_disable_ve() rather than opencoding it.

No practical change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>