KVM: selftests: Always open VM file descriptors with O_RDWR

Drop the @perm param from vm_create() and always open VM file descriptors
with O_RDWR.  There's no legitimate use case for other permissions, and
if a selftest wants to do oddball negative testing it can open code the
necessary bits instead of forcing a bunch of tests to provide useless
information.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>