x86/altp2m: Fix build with !CONFIG_HVM

x86/altp2m: Fix build with !CONFIG_HVM

c/s 0ec9b4ef3148 "x86/vmx: Fix security issue when a guest balloons out the #VE
info page" introduced a caller of altp2m_vcpu_disable_ve() in a common path,
but c/s e72ecc761541 "x86/altp2m: Rework #VE enable/disable paths" didn't have
a suitable prototype in the !CONFIG_HVM case.

Introduce one to fix the build.

Spotted by Travis.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>